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diff --git a/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_fit.json b/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_fit.json
new file mode 100644
index 0000000000000000000000000000000000000000..02e4e59bbd45e8e0a97c20a9b43501205c344c88
--- /dev/null
+++ b/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_fit.json
@@ -0,0 +1,64 @@
+{
+  "version": 1,
+  "M": 9,
+  "basis_function": "JaxB3",
+  "closed": true,
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+  "ribbon_width": 0.02500000037252903
+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_init.json b/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_init.json
new file mode 100644
index 0000000000000000000000000000000000000000..40db63b8e0f0b5388cd3c5ac8a930d156655580c
--- /dev/null
+++ b/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_init.json
@@ -0,0 +1,64 @@
+{
+  "version": 1,
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+  "ribbon_width": 0.02500000037252903
+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_stage1.json b/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_stage1.json
new file mode 100644
index 0000000000000000000000000000000000000000..decebb29731bd2970dbeefe740775b49433b29f6
--- /dev/null
+++ b/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_stage1.json
@@ -0,0 +1,64 @@
+{
+  "version": 1,
+  "M": 9,
+  "basis_function": "JaxB3",
+  "closed": true,
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+  "ribbon_width": 0.02500000037252903
+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/spline/twist_trajectory.jsonl b/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/spline/twist_trajectory.jsonl
new file mode 100644
index 0000000000000000000000000000000000000000..769b19b2b01b82ab1fcb38d45a9f42b7053701f5
--- /dev/null
+++ b/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/spline/twist_trajectory.jsonl
@@ -0,0 +1,10 @@
+{"iteration": 1, "loss": 9.14332576940069e-06, "raw_theta": [0.06470728840582513, 0.07366295577194364, 0.062430080734739705, 0.12150803971080443, 0.043448223632458836, -0.06363665195345061, -0.06667235297277414, -0.04641286354435361, -0.010984908507088859]}
+{"iteration": 2, "loss": 9.120336471823975e-06, "raw_theta": [0.07346226274820074, 0.05846728969559852, 0.024005731889442133, 0.15479125101295063, 0.04992806509041016, -0.08308090967754586, -0.07215765380293054, -0.06415460322996076, -0.02267835959201212]}
+{"iteration": 3, "loss": 9.102303010877222e-06, "raw_theta": [0.10319280718781768, 0.05528719918509254, -0.03649315699544299, 0.20724410289247722, 0.054036956415936394, -0.09941973426076717, -0.052312378182500584, -0.07864659289843627, -0.01521609513731055]}
+{"iteration": 4, "loss": 9.099274393520318e-06, "raw_theta": [0.09562429147570478, 0.07346850755753742, -0.06031870416993198, 0.2305485370778679, 0.040903715050341655, -0.1087750927100553, -0.02624245891200299, -0.09240436394954221, -0.03347943306057692]}
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+{"iteration": 10, "loss": 9.069818588613998e-06, "raw_theta": [0.05019233503263093, 0.11276737919002139, -0.08253360291051365, 0.2360001352937022, 0.05016859847459982, -0.1148893232259238, -0.011490561528414286, -0.11093424759099495, 0.004288341293795403]}
diff --git a/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/spline/witness.json b/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/spline/witness.json
new file mode 100644
index 0000000000000000000000000000000000000000..8388bb5c4c031df393401cc582cba93b7a2fb41c
--- /dev/null
+++ b/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/spline/witness.json
@@ -0,0 +1,22 @@
+{
+  "source": "synthetic upright elliptical band, four retained RP2 camera centres, both faces",
+  "seed": 6,
+  "noise_sigma_m": 0.0015,
+  "points": 288,
+  "ribbon_width_m": 0.025,
+  "truth_arclength_m": 0.664979607632856,
+  "fit_arclength_m": 0.6637027263641357,
+  "mean_point_to_ribbon_m": 0.0013682314893230796,
+  "mean_centerline_distance_to_truth_m": 0.0006032002581246964,
+  "max_centerline_distance_to_truth_m": 0.0015572715588724547,
+  "width_direction_abs_z_mean": 0.9987051767368632,
+  "width_direction_abs_z_min": 0.9958782713543136,
+  "theta_min_max_rad": [
+    -0.0679638460278511,
+    0.07198633253574371
+  ],
+  "rounds": 1,
+  "stop": "gradient_tolerance",
+  "twist_iterations": 12,
+  "twist_stop": "CONVERGENCE: RELATIVE REDUCTION OF F <= FACTR*EPSMCH"
+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/steps/colors.json b/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/steps/colors.json
new file mode 100644
index 0000000000000000000000000000000000000000..fe41262b3642d4d524ecdc5b7b3661832c90fbd0
--- /dev/null
+++ b/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/steps/colors.json
@@ -0,0 +1,67 @@
+{
+  "status": "ok",
+  "settings": {
+    "depth_tolerance_m": 0.025
+  },
+  "outputs": {
+    "rule": "native sRGB/255, cosine weighted over mask/depth/first-hit visible views",
+    "two_sided": true,
+    "depth_tolerance_m": 0.025,
+    "first_hit_tolerance_m": 0.0002,
+    "per_view": [
+      {
+        "view": 0,
+        "serial": "12540311",
+        "observed_vertices": 283,
+        "projected_inside": 450,
+        "first_hit": 371
+      },
+      {
+        "view": 1,
+        "serial": "12933256",
+        "observed_vertices": 327,
+        "projected_inside": 450,
+        "first_hit": 367
+      },
+      {
+        "view": 2,
+        "serial": "17736741",
+        "observed_vertices": 304,
+        "projected_inside": 450,
+        "first_hit": 376
+      },
+      {
+        "view": 3,
+        "serial": "23007103",
+        "observed_vertices": 339,
+        "projected_inside": 450,
+        "first_hit": 370
+      }
+    ],
+    "observed_vertices": 420,
+    "filled_vertices": 30,
+    "fill_color_rgb": [
+      0.40356320481659036,
+      0.34893722253995646,
+      0.2971836403655518
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+    "glb_roundtrip": {
+      "float32_positions_exact": true,
+      "vertex_colors_exact": true,
+      "uv_count": 0,
+      "vertex_reordered_on_open3d_load": true
+    },
+    "files": {
+      "colors/vertex_colors.npz": "18953e05fbbe6475b4f6351b8418ced79671cece7aa7e3812bddad41dbd0dd02",
+      "colors/color_summary.json": "46c5a3df0da3e5148c6f65b2aa95fe31a82eb3cafea30a00634eb05989c527e2",
+      "ribbon/shape_prior.glb": "58b0490fb60eda31a008e8c364e4e12a744af4f782da897fc26e8bf7a6ccc5b6",
+      "ribbon/ribbon.ply": "98f465f1185e7d2a8945c56377c6cd802c2e3d99db0479380754270aae2f47a5",
+      "gaussian_data/session_20260921_232242_960732/shape_prior.glb": "58b0490fb60eda31a008e8c364e4e12a744af4f782da897fc26e8bf7a6ccc5b6"
+    }
+  },
+  "code": {
+    "commit": "5e1b4258a3efb3620c533a4073a37212d8bf9ee5",
+    "u_pin": "81c718790a37e5e0102eb77af2c6edd34a9db25f"
+  },
+  "wall_time_s": 1.1939537525177002
+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/steps/diagnose-depth.json b/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/steps/diagnose-depth.json
new file mode 100644
index 0000000000000000000000000000000000000000..0d7b6fb680b18ed798cc7f7497a719c597a5770d
--- /dev/null
+++ b/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/steps/diagnose-depth.json
@@ -0,0 +1,110 @@
+{
+  "status": "ok",
+  "settings": {
+    "correction": "none",
+    "annulus_px": [
+      20,
+      80
+    ]
+  },
+  "outputs": {
+    "report": {
+      "annulus_px": [
+        20,
+        80
+      ],
+      "interior_erosion_px": 2,
+      "correction_applied": false,
+      "views": [
+        {
+          "view": 0,
+          "serial": "12540311",
+          "annulus_count": 71048,
+          "belt_interior_count": 8148,
+          "belt_edge_count": 3053,
+          "table_median_z_m": 0.00019330257288135977,
+          "belt_interior_median_z_m": 0.013223512045475005,
+          "belt_edge_median_z_m": 0.0013608664175257523,
+          "belt_minus_table_m": 0.013030209472593646,
+          "edge_minus_table_m": 0.0011675638446443926,
+          "table_p10_p90_z_m": [
+            -0.0010820395292243878,
+            0.0016168434599892702
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+          "belt_interior_p10_p90_z_m": [
+            0.001333146680920239,
+            0.024938806983344153
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+        },
+        {
+          "view": 1,
+          "serial": "12933256",
+          "annulus_count": 69649,
+          "belt_interior_count": 13168,
+          "belt_edge_count": 3376,
+          "table_median_z_m": -0.0008098168434317321,
+          "belt_interior_median_z_m": 0.011866238140904706,
+          "belt_edge_median_z_m": 0.003258902298937938,
+          "belt_minus_table_m": 0.012676054984336438,
+          "edge_minus_table_m": 0.00406871914236967,
+          "table_p10_p90_z_m": [
+            -0.0016696697493911784,
+            -0.000312581224704167
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+          "belt_interior_p10_p90_z_m": [
+            0.002009492652152345,
+            0.020916091420052456
+          ]
+        },
+        {
+          "view": 2,
+          "serial": "17736741",
+          "annulus_count": 84610,
+          "belt_interior_count": 9214,
+          "belt_edge_count": 3463,
+          "table_median_z_m": 0.00014563220284724565,
+          "belt_interior_median_z_m": 0.012193996336975266,
+          "belt_edge_median_z_m": 0.0018531337355257582,
+          "belt_minus_table_m": 0.01204836413412802,
+          "edge_minus_table_m": 0.0017075015326785126,
+          "table_p10_p90_z_m": [
+            -0.0007909478563196681,
+            0.0010656904976724675
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+          "belt_interior_p10_p90_z_m": [
+            0.0024343122896805635,
+            0.020218361151857715
+          ]
+        },
+        {
+          "view": 3,
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+          "belt_interior_count": 10680,
+          "belt_edge_count": 3205,
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+          "belt_interior_p10_p90_z_m": [
+            0.003380287681021821,
+            0.024217133935618224
+          ]
+        }
+      ]
+    },
+    "files": {
+      "inputs/depth_offset_diagnostic.json": "834dd5c23b32e746cfeb78e3bd2e42d66be8a5ca79565c51a24c21dd68790c55"
+    }
+  },
+  "code": {
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+    "u_pin": "81c718790a37e5e0102eb77af2c6edd34a9db25f"
+  },
+  "wall_time_s": 0.31661128997802734
+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/steps/fit.json b/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/steps/fit.json
new file mode 100644
index 0000000000000000000000000000000000000000..2c475d4554fa2f507bd7387b290da4ae4cc5f565
--- /dev/null
+++ b/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/steps/fit.json
@@ -0,0 +1,371 @@
+{
+  "status": "ok",
+  "settings": {
+    "object": {
+      "name": "grey_timing_belt",
+      "representation": "ribbon",
+      "fitter_source": "spline_estimation.timing_belt_fitting (scratch/test_timing_belt_perception.py)",
+      "dimension_source": "demonstrated 25 mm width; surface only, no interior (corpus Q1, 2026-10-25)",
+      "radius_m": null,
+      "ribbon_width_m": 0.025,
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+      "step_angle_deg": null,
+      "u_resolution": 5,
+      "overlap_rule": null,
+      "twist_penalty": {
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+        "threshold_per_m": 10.0,
+        "huber_per_m": 30.0,
+        "cap_per_m": 80.0,
+        "weight": 5e-08
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+    },
+    "max_rounds": 400,
+    "relative_loss_tolerance": 1e-07,
+    "reprojection": {
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+      "direction": "visible mask pixels to projected centerline",
+      "ribbon_twist_stage": "two-sided (owner Q11): visible mask pixels to projected width segments plus visible projected surface samples (off the occlusion union) to the nearest mask pixel",
+      "mask": "accepted object outside hand/occluder union",
+      "view_aggregation": "equal view means",
+      "distance": "metric image plane using per-view median visible belt depth and fx/fy",
+      "samples": 360,
+      "huber_delta_m": 0.005,
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+      "baseline_run": null
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+    "loss_config": {
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+      "max_saturation_distance": 0.035,
+      "distance_huber_delta": 0.005,
+      "eval_curvature_cost": true,
+      "num_eval_reg_points": 250,
+      "lambda_curvature_cost": 5e-08,
+      "max_curvature": 10.0,
+      "curvature_huber_delta": 30.0,
+      "max_saturation_curvature": 80.0,
+      "eval_curvilinear_energy": false,
+      "lambda_curvilinear_energy": 0.001,
+      "eval_length_constraint": true,
+      "lambda_length_constraint": 0.1,
+      "arclength_bounds": [
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+        0.6660000085830688
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+    },
+    "solver_config": {
+      "solver_type": "lbfgs",
+      "linesearch_type": "zoom",
+      "max_linesearch_steps": 10,
+      "grad_norm_tolerance": 0.0003,
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+    },
+    "ribbon_twist_refinement": {
+      "stage": "2 of 2; centerline control points frozen at stage 1, nine raw twist knots free",
+      "start": "stage-1 knots reduced modulo pi to bounded twist in (-pi/2, pi/2]",
+      "loss": "3D ribbon distance + weight x two-sided surface mask term + twist penalty",
+      "twist_penalty": {
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+    }
+  },
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+          "kept": 12836,
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+        },
+        {
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+    "partial_view_witness": {
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+      "seed": 6,
+      "noise_sigma_m": 0.0015,
+      "points": 288,
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+      "twist_stop": "CONVERGENCE: RELATIVE REDUCTION OF F <= FACTR*EPSMCH"
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+    "stage1_final": {
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+      "stop": "CONVERGENCE: RELATIVE REDUCTION OF F <= FACTR*EPSMCH",
+      "success": true
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+      "spline/init_ellipse.json": "8c716d81ad9657f334cfa04da7148df905f4f26761c48e72a0b0d6ab369f6e79",
+      "spline/reprojection.json": "ed35c48ce23e8d8cde4c3ee9e4108a382a7e8950bb24d24436a246261444685b"
+    }
+  },
+  "code": {
+    "commit": "5e1b4258a3efb3620c533a4073a37212d8bf9ee5",
+    "u_pin": "81c718790a37e5e0102eb77af2c6edd34a9db25f"
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+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/steps/hybrid.json b/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/steps/hybrid.json
new file mode 100644
index 0000000000000000000000000000000000000000..f2c44e8a78cb1a6e17b26b9f6980380c9f647300
--- /dev/null
+++ b/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/steps/hybrid.json
@@ -0,0 +1,24 @@
+{
+  "status": "ok",
+  "settings": {
+    "u_pin": "81c718790a37e5e0102eb77af2c6edd34a9db25f",
+    "numpy_seed": 0,
+    "points_per_triangle": 30
+  },
+  "outputs": {
+    "counts": {
+      "observed": 39841,
+      "mesh": 21600,
+      "total": 61441,
+      "mesh_to_observed": 0.5421550663888959
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+    "files": {
+      "hybrid/populations.npz": "129e4c1f5a238ff44eaa69c928ad20e4ff0a4117a0db16d3c1ed54c1977025af"
+    }
+  },
+  "code": {
+    "commit": "5e1b4258a3efb3620c533a4073a37212d8bf9ee5",
+    "u_pin": "81c718790a37e5e0102eb77af2c6edd34a9db25f"
+  },
+  "wall_time_s": 0.8793861865997314
+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/steps/inputs.json b/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/steps/inputs.json
new file mode 100644
index 0000000000000000000000000000000000000000..ac5db85de3a5bf35517007f8ae9e9d9a19ad0912
--- /dev/null
+++ b/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/steps/inputs.json
@@ -0,0 +1,216 @@
+{
+  "status": "ok",
+  "settings": {
+    "object": "grey_timing_belt",
+    "depth_source": "waft_stereo_dav2l5",
+    "depth_stream_ids": [
+      "zed_12540311_depth_waft_stereo_synlarge_dav2l5_zdepth",
+      "zed_12933256_depth_waft_stereo_synlarge_dav2l5_zdepth",
+      "zed_17736741_depth_waft_stereo_synlarge_dav2l5_zdepth",
+      "zed_23007103_depth_waft_stereo_synlarge_dav2l5_zdepth"
+    ],
+    "frames": "/mnt/data2/magna/belt_perception_shared/phystwin_model_corpus_expansion/lab/session_20260921_232242_960732/window_anchor0307-0584/accepted_input_preparation_c6a_v1/frames_waft/per_camera_inputs.json",
+    "frames_sha256": "627b4281b4d20f4ccbef9557989d00f5da058ff139662ffd747a44a912b3ab2d",
+    "signed_native_frames_sha256": {
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+      "17736741": "5ad58064ab6c1d56142cb8773004dca6ff4854e0150d9a1f61d2c241afd6a660",
+      "23007103": "3a428f0c33241b381af725746891d72ced5620793d14a0994c3eb7906f4348d8"
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+      "17736741": "382b607139b102495b94b89c9aa91da03686812ae26cc9a4309fd63ac4d09f25",
+      "23007103": "18d595727d05d81427303e2aed35b10fdc041105879694509c8b1bb2963c2507"
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+    "filter": {
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+      "input_count": 41210,
+      "kept_count": 39841,
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diff --git a/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/steps/review.json b/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/steps/review.json
new file mode 100644
index 0000000000000000000000000000000000000000..257f5e23b2512779e0b1e4304a0f3a01cea244b6
--- /dev/null
+++ b/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/steps/review.json
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\ No newline at end of file
diff --git a/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/steps/ribbon.json b/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/steps/ribbon.json
new file mode 100644
index 0000000000000000000000000000000000000000..d32ea9a8a36eb70e750dfe19c356764661e90ca0
--- /dev/null
+++ b/sessions/session_20260921_232242_960732/preprocessing/timing-belt-ribbon-prior-v1/native/steps/ribbon.json
@@ -0,0 +1,71 @@
+{
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+    "step_t": 0.1,
+    "u_resolution": 5,
+    "interior": "none (owner surface-only route)"
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+  "outputs": {
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+      "euler_characteristic": 0,
+      "watertight": false,
+      "components": 1,
+      "winding_consistent": true,
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+      "confirmed_intersecting_pairs": [],
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+}
\ No newline at end of file