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diff --git a/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_fit.json b/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_fit.json
new file mode 100644
index 0000000000000000000000000000000000000000..d05a297c5cabb083988462d19a62982908177166
--- /dev/null
+++ b/sessions/session_20260921_232717_273624/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_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_init.json b/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_init.json
new file mode 100644
index 0000000000000000000000000000000000000000..ffe5d1baeff09b631f47a59eb5d598e9aded5d5f
--- /dev/null
+++ b/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_init.json
@@ -0,0 +1,64 @@
+{
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+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_stage1.json b/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_stage1.json
new file mode 100644
index 0000000000000000000000000000000000000000..e96798a3ef7334cc3dd62b4659aef89559036c32
--- /dev/null
+++ b/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_stage1.json
@@ -0,0 +1,64 @@
+{
+  "version": 1,
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+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/spline/twist_trajectory.jsonl b/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/spline/twist_trajectory.jsonl
new file mode 100644
index 0000000000000000000000000000000000000000..1b6e58b1f72f101ef8700132ccb6b57e31b8dc74
--- /dev/null
+++ b/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/spline/twist_trajectory.jsonl
@@ -0,0 +1,12 @@
+{"iteration": 1, "loss": 1.108085416490212e-05, "raw_theta": [0.06539012392132301, 0.09498797543888739, 0.02803581280594191, 0.05774431753761436, 0.11402308350940019, 0.07709350123892571, -0.032807566995860994, -0.04736596930748611, 0.017596564709750365]}
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+{"iteration": 12, "loss": 1.0926227332674898e-05, "raw_theta": [0.09217562548540317, 0.2786951581587835, -0.080356634740689, 0.01580890912566483, 0.2728998971241382, 0.02640458322132521, -0.08258852307592453, -0.03531005281445597, -0.025332106134316475]}
diff --git a/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/steps/colors.json b/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/steps/colors.json
new file mode 100644
index 0000000000000000000000000000000000000000..718d772a321d3e38525b9e0d468aa4ddc3dc22a1
--- /dev/null
+++ b/sessions/session_20260921_232717_273624/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": 309,
+        "projected_inside": 450,
+        "first_hit": 376
+      },
+      {
+        "view": 1,
+        "serial": "12933256",
+        "observed_vertices": 180,
+        "projected_inside": 450,
+        "first_hit": 363
+      },
+      {
+        "view": 2,
+        "serial": "17736741",
+        "observed_vertices": 325,
+        "projected_inside": 450,
+        "first_hit": 383
+      },
+      {
+        "view": 3,
+        "serial": "23007103",
+        "observed_vertices": 305,
+        "projected_inside": 450,
+        "first_hit": 369
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+    ],
+    "observed_vertices": 405,
+    "filled_vertices": 45,
+    "fill_color_rgb": [
+      0.4130485070194236,
+      0.3501026043110393,
+      0.2935920691021704
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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": "22a99b7189a112ff6be26e49bf4c0aa5f4593d6fa81ca1101d2f988d1dcf219b",
+      "colors/color_summary.json": "b3293b44b56a07de51ac696a55081c327fe182154f8ece6a43720c72d94a9f0f",
+      "ribbon/shape_prior.glb": "f689ceecf96d013b7f3e7ba6e03aa08b006339797651f415ece332dd7eaaef61",
+      "ribbon/ribbon.ply": "815df146a21966028054c48e5cbf66cd88c9fb3aa71dda2924f5514d745e2142",
+      "gaussian_data/session_20260921_232717_273624/shape_prior.glb": "f689ceecf96d013b7f3e7ba6e03aa08b006339797651f415ece332dd7eaaef61"
+    }
+  },
+  "code": {
+    "commit": "d1d2f1381470138dfbb3c58d448e53bfed1e0d7e",
+    "u_pin": "81c718790a37e5e0102eb77af2c6edd34a9db25f"
+  },
+  "wall_time_s": 1.1542484760284424
+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/steps/diagnose-depth.json b/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/steps/diagnose-depth.json
new file mode 100644
index 0000000000000000000000000000000000000000..4fe4272dd38f567cac0340219e34fd4cfe042e6d
--- /dev/null
+++ b/sessions/session_20260921_232717_273624/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": [
+        {
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+          "serial": "12540311",
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+          "belt_interior_count": 9223,
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+          "belt_minus_table_m": 0.014480599054810694,
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+          "table_p10_p90_z_m": [
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+            0.0012174695495640777
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+          "belt_interior_p10_p90_z_m": [
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+        {
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+          "serial": "12933256",
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+          "belt_interior_count": 7516,
+          "belt_edge_count": 1962,
+          "table_median_z_m": -0.0007741575156395841,
+          "belt_interior_median_z_m": 0.009729761751823385,
+          "belt_edge_median_z_m": 0.006033844598657412,
+          "belt_minus_table_m": 0.010503919267462969,
+          "edge_minus_table_m": 0.006808002114296996,
+          "table_p10_p90_z_m": [
+            -0.0012739911649638723,
+            -7.552693662037384e-05
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+          "belt_interior_p10_p90_z_m": [
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+            0.018587164113242904
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+        {
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+          "serial": "17736741",
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+          "belt_interior_count": 10137,
+          "belt_edge_count": 3780,
+          "table_median_z_m": 0.0003272291734691124,
+          "belt_interior_median_z_m": 0.009117033255995,
+          "belt_edge_median_z_m": 0.001996162379860611,
+          "belt_minus_table_m": 0.008789804082525887,
+          "edge_minus_table_m": 0.0016689332063914986,
+          "table_p10_p90_z_m": [
+            -0.0007368257105135334,
+            0.0015993097291488014
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+          "belt_interior_p10_p90_z_m": [
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+            0.021592085147589805
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+        },
+        {
+          "view": 3,
+          "serial": "23007103",
+          "annulus_count": 52063,
+          "belt_interior_count": 9063,
+          "belt_edge_count": 2685,
+          "table_median_z_m": 0.0005592939767602445,
+          "belt_interior_median_z_m": 0.015321748844323801,
+          "belt_edge_median_z_m": 0.003449738670074032,
+          "belt_minus_table_m": 0.014762454867563557,
+          "edge_minus_table_m": 0.0028904446933137873,
+          "table_p10_p90_z_m": [
+            -0.00013165901367675472,
+            0.0010480366872363644
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+          "belt_interior_p10_p90_z_m": [
+            0.005199978022473495,
+            0.02446652005252604
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+        }
+      ]
+    },
+    "files": {
+      "inputs/depth_offset_diagnostic.json": "f5fa661de64c3f65665f0babb019590fe6a4832622e3efad72cb3ad83f647b76"
+    }
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+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/steps/fit.json b/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/steps/fit.json
new file mode 100644
index 0000000000000000000000000000000000000000..d604b6898d0f913c7fbba909c846f87fdc4a8c48
--- /dev/null
+++ b/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/steps/fit.json
@@ -0,0 +1,372 @@
+{
+  "status": "ok",
+  "settings": {
+    "object": {
+      "name": "grey_timing_belt",
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+      "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,
+      "step_t": 0.1,
+      "step_angle_deg": null,
+      "u_resolution": 5,
+      "overlap_rule": null,
+      "twist_penalty": {
+        "samples": 250,
+        "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,
+    "control_count": 9,
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+      "weight": 4.0,
+      "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,
+      "smooth_cap_m": 0.035,
+      "baseline_run": null
+    },
+    "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,
+      "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": [
+        0.6639999747276306,
+        0.6660000085830688
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+    },
+    "solver_config": {
+      "solver_type": "lbfgs",
+      "linesearch_type": "zoom",
+      "max_linesearch_steps": 10,
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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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+    "deterministic": true,
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+      "seed": 6,
+      "noise_sigma_m": 0.0015,
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+      "stop": "gradient_tolerance",
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+      "twist_stop": "CONVERGENCE: RELATIVE REDUCTION OF F <= FACTR*EPSMCH"
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+    "stage1_final": {
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+      "spline/reprojection.json": "2b1fa0d175c37494724e76b5f42cb28cbf585d22efffb90845ba199428317dde"
+    }
+  },
+  "code": {
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+    "u_pin": "81c718790a37e5e0102eb77af2c6edd34a9db25f"
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+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/steps/hybrid.json b/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/steps/hybrid.json
new file mode 100644
index 0000000000000000000000000000000000000000..3ede63e3a8554d92955415b09d22f5952b90dc53
--- /dev/null
+++ b/sessions/session_20260921_232717_273624/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": 34798,
+      "mesh": 21600,
+      "total": 56398,
+      "mesh_to_observed": 0.6207253290418989
+    },
+    "files": {
+      "hybrid/populations.npz": "7e811b619bb1ae41157a88bf6a2e9e48c6864fd6b3cf19e3b958010afd1519d6"
+    }
+  },
+  "code": {
+    "commit": "d1d2f1381470138dfbb3c58d448e53bfed1e0d7e",
+    "u_pin": "81c718790a37e5e0102eb77af2c6edd34a9db25f"
+  },
+  "wall_time_s": 0.7851665019989014
+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/steps/inputs.json b/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/steps/inputs.json
new file mode 100644
index 0000000000000000000000000000000000000000..7270d2e6a3914410c44c42dd920377d978f03d80
--- /dev/null
+++ b/sessions/session_20260921_232717_273624/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_232717_273624/window_anchor0340-0691/accepted_input_preparation_c6a_v1/frames_waft/per_camera_inputs.json",
+    "frames_sha256": "6bf8b4e86c9b020d202eda2fd447e94404522cbaac61ed9235731dbff43f9459",
+    "signed_native_frames_sha256": {
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+      "17736741": "30d7e802a088889abdd6b434a617c24282d3ffa08c6b1cf932790c9467122d04",
+      "23007103": "1681338d7e447a78359cea139a03115ce732121b540a86eedce2948167bf0e5f"
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+    "filter": {
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diff --git a/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/steps/review.json b/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/steps/review.json
new file mode 100644
index 0000000000000000000000000000000000000000..a8ba6150dad1ee045e31b1c218634dbed01043b8
--- /dev/null
+++ b/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/steps/review.json
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+{
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+    "historical_sha256": null,
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+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/steps/ribbon.json b/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/steps/ribbon.json
new file mode 100644
index 0000000000000000000000000000000000000000..14eb53203110bdccc6a017a17496f8c7e8c1120c
--- /dev/null
+++ b/sessions/session_20260921_232717_273624/preprocessing/timing-belt-ribbon-prior-v1/native/steps/ribbon.json
@@ -0,0 +1,71 @@
+{
+  "status": "ok",
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+    "step_t": 0.1,
+    "u_resolution": 5,
+    "interior": "none (owner surface-only route)"
+  },
+  "outputs": {
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+      "watertight": false,
+      "components": 1,
+      "winding_consistent": true,
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+      "confirmed_intersecting_pairs": [],
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+}
\ No newline at end of file