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diff --git a/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_fit.json b/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_fit.json
new file mode 100644
index 0000000000000000000000000000000000000000..d6be59dffa1b4a2dad51f08e24d58e8d3698fc24
--- /dev/null
+++ b/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_fit.json
@@ -0,0 +1,64 @@
+{
+  "version": 1,
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+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_init.json b/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_init.json
new file mode 100644
index 0000000000000000000000000000000000000000..180e2ece43c1afd1dd8653db46c0624086450a6f
--- /dev/null
+++ b/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_init.json
@@ -0,0 +1,64 @@
+{
+  "version": 1,
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+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_stage1.json b/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_stage1.json
new file mode 100644
index 0000000000000000000000000000000000000000..707f30affb17359d9a55f9ba4d8dff971773a543
--- /dev/null
+++ b/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_stage1.json
@@ -0,0 +1,64 @@
+{
+  "version": 1,
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+  "closed": true,
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+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/spline/twist_trajectory.jsonl b/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/spline/twist_trajectory.jsonl
new file mode 100644
index 0000000000000000000000000000000000000000..bd703566973dbb5184723ef8cb2207d151e54baf
--- /dev/null
+++ b/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/spline/twist_trajectory.jsonl
@@ -0,0 +1,9 @@
+{"iteration": 1, "loss": 1.1508553143357858e-05, "raw_theta": [0.056532521084766446, 0.04987111782685847, -0.06636345710311732, 0.038877507846740635, 0.064793369487112, 0.040791253142897714, -0.03557738241467719, 0.004427087288803439, 0.008429426299643217]}
+{"iteration": 2, "loss": 1.1306368833174929e-05, "raw_theta": [0.15762447993284345, 0.14845066673287338, -0.20618722213710902, 0.12182105335626522, 0.09196011961989191, 0.03752397799797221, -0.11559230093115586, 0.015484204051209507, -0.010206364036541724]}
+{"iteration": 3, "loss": 1.1280782018729951e-05, "raw_theta": [0.24611795754082413, 0.2206782356726339, -0.36974975093227935, 0.28632065854115396, 0.061841277373631834, 0.06584776069466153, -0.10132255113308838, 0.08524795998089704, -0.07510256853094026]}
+{"iteration": 4, "loss": 1.1275542419753037e-05, "raw_theta": [0.24542117594179438, 0.22079083703520358, -0.3720565747316113, 0.2850162580605953, 0.05457831673739387, 0.06507203236348712, -0.1041144222918048, 0.08818981700988376, -0.07530846670945966]}
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+{"iteration": 9, "loss": 1.1237652870477177e-05, "raw_theta": [0.224350051514281, 0.2225316865171331, -0.3734981744049238, 0.2933927922175766, -0.01219774591007451, 0.08016144746182038, -0.1304775246979825, 0.12419607937912537, -0.08838957842341862]}
diff --git a/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/spline/witness.json b/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/spline/witness.json
new file mode 100644
index 0000000000000000000000000000000000000000..bcaeeb9141eacb30d89999264e01d10e97830801
--- /dev/null
+++ b/sessions/session_20260921_232344_289067/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": 0.6637030839920044,
+  "mean_point_to_ribbon_m": 0.001368213794194162,
+  "mean_centerline_distance_to_truth_m": 0.0006031105826376801,
+  "max_centerline_distance_to_truth_m": 0.0015569646493600495,
+  "width_direction_abs_z_mean": 0.9987052460417335,
+  "width_direction_abs_z_min": 0.9958789046536549,
+  "theta_min_max_rad": [
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+    0.07201700657606125
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+  "rounds": 1,
+  "stop": "gradient_tolerance",
+  "twist_iterations": 10,
+  "twist_stop": "CONVERGENCE: RELATIVE REDUCTION OF F <= FACTR*EPSMCH"
+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/steps/colors.json b/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/steps/colors.json
new file mode 100644
index 0000000000000000000000000000000000000000..02c947bfdb95a741734ce188cb5a7c6564276894
--- /dev/null
+++ b/sessions/session_20260921_232344_289067/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": 263,
+        "projected_inside": 450,
+        "first_hit": 379
+      },
+      {
+        "view": 1,
+        "serial": "12933256",
+        "observed_vertices": 176,
+        "projected_inside": 450,
+        "first_hit": 362
+      },
+      {
+        "view": 2,
+        "serial": "17736741",
+        "observed_vertices": 300,
+        "projected_inside": 450,
+        "first_hit": 365
+      },
+      {
+        "view": 3,
+        "serial": "23007103",
+        "observed_vertices": 285,
+        "projected_inside": 450,
+        "first_hit": 375
+      }
+    ],
+    "observed_vertices": 393,
+    "filled_vertices": 57,
+    "fill_color_rgb": [
+      0.40392156862745093,
+      0.34509803921568627,
+      0.2923340622683475
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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": "e326dc6cfb203b81cd5dd564704736f77dd3341d714e2cfab9e3f3407dcb16c9",
+      "colors/color_summary.json": "68edc9ac2d34e8343362f0bab9a37a0120b64c128c6b35cc4fe95a5269ab61d7",
+      "ribbon/shape_prior.glb": "a2a9728d596afe59e4da785c0d1c60c54cbefc7b19c69fff2582496a7357ba15",
+      "ribbon/ribbon.ply": "3ada8d9161bcd2d6b92489e46af8701d1547a96588537117bfa948a8bf9f015a",
+      "gaussian_data/session_20260921_232344_289067/shape_prior.glb": "a2a9728d596afe59e4da785c0d1c60c54cbefc7b19c69fff2582496a7357ba15"
+    }
+  },
+  "code": {
+    "commit": "5e1b4258a3efb3620c533a4073a37212d8bf9ee5",
+    "u_pin": "81c718790a37e5e0102eb77af2c6edd34a9db25f"
+  },
+  "wall_time_s": 1.210578203201294
+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/steps/diagnose-depth.json b/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/steps/diagnose-depth.json
new file mode 100644
index 0000000000000000000000000000000000000000..4cbe435d4ffc4f59b6849a3810e0423c1624b089
--- /dev/null
+++ b/sessions/session_20260921_232344_289067/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": 74148,
+          "belt_interior_count": 8280,
+          "belt_edge_count": 3252,
+          "table_median_z_m": -4.352607307039902e-05,
+          "belt_interior_median_z_m": 0.012525813068355651,
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+          "belt_minus_table_m": 0.01256933914142605,
+          "edge_minus_table_m": 0.000914855414855198,
+          "table_p10_p90_z_m": [
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+            0.0011985741059319047
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+          "belt_interior_p10_p90_z_m": [
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+            0.026493068694722913
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+        },
+        {
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+          "serial": "12933256",
+          "annulus_count": 43588,
+          "belt_interior_count": 7839,
+          "belt_edge_count": 1977,
+          "table_median_z_m": -0.0003382319759696051,
+          "belt_interior_median_z_m": 0.008282491058796193,
+          "belt_edge_median_z_m": 0.002746544959047248,
+          "belt_minus_table_m": 0.008620723034765798,
+          "edge_minus_table_m": 0.003084776935016853,
+          "table_p10_p90_z_m": [
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+            0.00020287459226578968
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+          "belt_interior_p10_p90_z_m": [
+            0.0003070594801364292,
+            0.018209743744855643
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+        {
+          "view": 2,
+          "serial": "17736741",
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+          "belt_interior_count": 9442,
+          "belt_edge_count": 3802,
+          "table_median_z_m": 0.0022106602226864003,
+          "belt_interior_median_z_m": 0.007650337124667672,
+          "belt_edge_median_z_m": 0.003727536878046278,
+          "belt_minus_table_m": 0.005439676901981272,
+          "edge_minus_table_m": 0.0015168766553598778,
+          "table_p10_p90_z_m": [
+            0.0012418981287883103,
+            0.0030986497468127984
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+          "belt_interior_p10_p90_z_m": [
+            0.0041037672150814105,
+            0.01880508164753212
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+        },
+        {
+          "view": 3,
+          "serial": "23007103",
+          "annulus_count": 54569,
+          "belt_interior_count": 9524,
+          "belt_edge_count": 2797,
+          "table_median_z_m": 0.0005609506453687629,
+          "belt_interior_median_z_m": 0.012464615933365453,
+          "belt_edge_median_z_m": 0.0017488104713860864,
+          "belt_minus_table_m": 0.01190366528799669,
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+          "belt_interior_p10_p90_z_m": [
+            0.0035697442340827724,
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+          ]
+        }
+      ]
+    },
+    "files": {
+      "inputs/depth_offset_diagnostic.json": "4d853c031e598c5ea500e6e3c03db86fbf6ca8e5838db04dd579d7fd8b1ee8f5"
+    }
+  },
+  "code": {
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+    "u_pin": "81c718790a37e5e0102eb77af2c6edd34a9db25f"
+  },
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+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/steps/fit.json b/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/steps/fit.json
new file mode 100644
index 0000000000000000000000000000000000000000..77b0b1e1c6efdec37923fff1140fb9efbbcf9815
--- /dev/null
+++ b/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/steps/fit.json
@@ -0,0 +1,371 @@
+{
+  "status": "ok",
+  "settings": {
+    "object": {
+      "name": "grey_timing_belt",
+      "representation": "ribbon",
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+      "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,
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+    "max_rounds": 400,
+    "relative_loss_tolerance": 1e-07,
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+      "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,
+      "smooth_cap_m": 0.035,
+      "baseline_run": null
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+    "loss_config": {
+      "eval_distances": true,
+      "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,
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+      "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",
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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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+        "threshold_per_m": 10.0,
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+    }
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+          "live_20_0.05_kept": 3319,
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+      "seed": 6,
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+      "points": 288,
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+      "twist_stop": "CONVERGENCE: RELATIVE REDUCTION OF F <= FACTR*EPSMCH"
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+      "spline/rounds.jsonl": "2695a124b9f56d7a29c3eb8ff0581a4c0487adfc284469f5274b04b24cfb169e",
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+      "spline/reprojection.json": "a7e01a54204132331b126ce358baf7d1535667aaec7605b8101dbed951e98273"
+    }
+  },
+  "code": {
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+    "u_pin": "81c718790a37e5e0102eb77af2c6edd34a9db25f"
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+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/steps/hybrid.json b/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/steps/hybrid.json
new file mode 100644
index 0000000000000000000000000000000000000000..01fbed0bc6d7687a64c1d58b769ef98257030a23
--- /dev/null
+++ b/sessions/session_20260921_232344_289067/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": 33871,
+      "mesh": 21600,
+      "total": 55471,
+      "mesh_to_observed": 0.6377136783679254
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+    "files": {
+      "hybrid/populations.npz": "9086802738c5ea3b05a5f56ad622e5775657e6b88163542d1acd0642a6a24e66"
+    }
+  },
+  "code": {
+    "commit": "5e1b4258a3efb3620c533a4073a37212d8bf9ee5",
+    "u_pin": "81c718790a37e5e0102eb77af2c6edd34a9db25f"
+  },
+  "wall_time_s": 0.861283540725708
+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/steps/inputs.json b/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/steps/inputs.json
new file mode 100644
index 0000000000000000000000000000000000000000..914820cace453370b55c8364971cf3771eb4ec11
--- /dev/null
+++ b/sessions/session_20260921_232344_289067/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": [
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+      "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_232344_289067/window_anchor0308-0680/accepted_input_preparation_c6a_v1/frames_waft/per_camera_inputs.json",
+    "frames_sha256": "f4fd3f4818c969d7515a3ce877fbb11f2f276034b83a94d405ded4c9471e08f6",
+    "signed_native_frames_sha256": {
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+      "17736741": "743b2d551f6c9ddf613c42a3eb51c486307f4e462c3833757c8d13c3bf67a651",
+      "23007103": "90b0b56b561de50e8f26c0ad2140fad7aa30af54af7916d6d5b2327a098ba8f5"
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+    "filter": {
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diff --git a/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/steps/review.json b/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/steps/review.json
new file mode 100644
index 0000000000000000000000000000000000000000..3bec787b7b59fd482b390eaf2172976e039b51de
--- /dev/null
+++ b/sessions/session_20260921_232344_289067/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_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/steps/ribbon.json b/sessions/session_20260921_232344_289067/preprocessing/timing-belt-ribbon-prior-v1/native/steps/ribbon.json
new file mode 100644
index 0000000000000000000000000000000000000000..87bf94968ac527a7f2041042eec676a84d95ca6f
--- /dev/null
+++ b/sessions/session_20260921_232344_289067/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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+      "watertight": false,
+      "components": 1,
+      "winding_consistent": true,
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+}
\ No newline at end of file