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diff --git a/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_fit.json b/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_fit.json
new file mode 100644
index 0000000000000000000000000000000000000000..15fbc4d28ec837bef706070f6a5b43551612813c
--- /dev/null
+++ b/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_fit.json
@@ -0,0 +1,64 @@
+{
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+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_init.json b/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_init.json
new file mode 100644
index 0000000000000000000000000000000000000000..f2265a675a79183779567141861394352b18a3a3
--- /dev/null
+++ b/sessions/session_20260921_232616_062985/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_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_stage1.json b/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_stage1.json
new file mode 100644
index 0000000000000000000000000000000000000000..c5a4323eb63e29f2269db84ed8c8b33d42aa414f
--- /dev/null
+++ b/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/spline/spline_stage1.json
@@ -0,0 +1,64 @@
+{
+  "version": 1,
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+  "basis_function": "JaxB3",
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+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/spline/twist_trajectory.jsonl b/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/spline/twist_trajectory.jsonl
new file mode 100644
index 0000000000000000000000000000000000000000..945fb449d72c319e373d77a83c7a474934f31417
--- /dev/null
+++ b/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/spline/twist_trajectory.jsonl
@@ -0,0 +1,8 @@
+{"iteration": 1, "loss": 1.368627908959752e-05, "raw_theta": [-0.017524975469305756, 0.017392992599501216, 0.039468719714553444, 0.08190446867505512, 0.05127072163922695, 0.019753120156219106, -0.04385248321382388, -0.04239350582826234, -0.08200607039485247]}
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+{"iteration": 4, "loss": 1.3558301361626945e-05, "raw_theta": [0.030224937069548402, 0.03827596175239348, -0.053512904191319224, 0.3444569888823305, -0.0872198960943924, 0.11535597779267638, -0.19039239256460527, 0.056658501176416194, -0.13633308482890824]}
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+{"iteration": 6, "loss": 1.3520582797355019e-05, "raw_theta": [0.045412504854612915, 0.05473320494935985, -0.09494859555615064, 0.3714587390936761, -0.1048177365888161, 0.1742398145862152, -0.21880488505325119, 0.08530917769677439, -0.13568438552471335]}
+{"iteration": 7, "loss": 1.3520580978365615e-05, "raw_theta": [0.04540999184320625, 0.0547333163858044, -0.09495211512006882, 0.37145428042772377, -0.10482258042712216, 0.17423903415187011, -0.2188056806888908, 0.0853088041622307, -0.13568741106835422]}
+{"iteration": 8, "loss": 1.3520580978365615e-05, "raw_theta": [0.045409991842457, 0.05473331638605315, -0.09495211512128858, 0.37145428042663037, -0.1048225804281494, 0.17423903415200956, -0.21880568068913386, 0.08530880416227932, -0.13568741106915116]}
diff --git a/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/steps/colors.json b/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/steps/colors.json
new file mode 100644
index 0000000000000000000000000000000000000000..1780307eec1ed85bac8b90fe94e4d557fe3fe754
--- /dev/null
+++ b/sessions/session_20260921_232616_062985/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": 305,
+        "projected_inside": 450,
+        "first_hit": 378
+      },
+      {
+        "view": 1,
+        "serial": "12933256",
+        "observed_vertices": 166,
+        "projected_inside": 450,
+        "first_hit": 382
+      },
+      {
+        "view": 2,
+        "serial": "17736741",
+        "observed_vertices": 307,
+        "projected_inside": 450,
+        "first_hit": 370
+      },
+      {
+        "view": 3,
+        "serial": "23007103",
+        "observed_vertices": 286,
+        "projected_inside": 450,
+        "first_hit": 359
+      }
+    ],
+    "observed_vertices": 410,
+    "filled_vertices": 40,
+    "fill_color_rgb": [
+      0.39801180218044463,
+      0.3428367832867175,
+      0.29473908886040706
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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": "479c243b8c068fa1a901771f8b7595e1f79ffda897522007c66c1c7816862511",
+      "colors/color_summary.json": "11ebb7e4dfea61d80fbc9c5f8ee5533ef2708f7d541c61cf9f886de7353dc3cb",
+      "ribbon/shape_prior.glb": "158798dc8b7619454e03c87bf79325e174d7a681b09fe58f6263676529932114",
+      "ribbon/ribbon.ply": "3b11a26a3328b3d4e0c28effc7381ed24fad564f90fc8a04f41231c7d555a521",
+      "gaussian_data/session_20260921_232616_062985/shape_prior.glb": "158798dc8b7619454e03c87bf79325e174d7a681b09fe58f6263676529932114"
+    }
+  },
+  "code": {
+    "commit": "d1d2f1381470138dfbb3c58d448e53bfed1e0d7e",
+    "u_pin": "81c718790a37e5e0102eb77af2c6edd34a9db25f"
+  },
+  "wall_time_s": 1.2119371891021729
+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/steps/diagnose-depth.json b/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/steps/diagnose-depth.json
new file mode 100644
index 0000000000000000000000000000000000000000..462d3ddad4afbc1fdbe24aefec8ab8c5ff2a65c0
--- /dev/null
+++ b/sessions/session_20260921_232616_062985/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": 79139,
+          "belt_interior_count": 8962,
+          "belt_edge_count": 3584,
+          "table_median_z_m": 0.00046781750874125283,
+          "belt_interior_median_z_m": 0.014186149409212812,
+          "belt_edge_median_z_m": 0.0029567844914553665,
+          "belt_minus_table_m": 0.013718331900471559,
+          "edge_minus_table_m": 0.0024889669827141137,
+          "table_p10_p90_z_m": [
+            -0.0008852931250295848,
+            0.001860860225569038
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+          "belt_interior_p10_p90_z_m": [
+            0.003239400929596903,
+            0.026881318370467425
+          ]
+        },
+        {
+          "view": 1,
+          "serial": "12933256",
+          "annulus_count": 39455,
+          "belt_interior_count": 7447,
+          "belt_edge_count": 1911,
+          "table_median_z_m": -0.0009817749412076582,
+          "belt_interior_median_z_m": 0.009068062242887365,
+          "belt_edge_median_z_m": 0.00709150520810331,
+          "belt_minus_table_m": 0.010049837184095023,
+          "edge_minus_table_m": 0.008073280149310968,
+          "table_p10_p90_z_m": [
+            -0.002139528024902937,
+            7.848556485341436e-05
+          ],
+          "belt_interior_p10_p90_z_m": [
+            0.0016286709874211215,
+            0.019644991723526076
+          ]
+        },
+        {
+          "view": 2,
+          "serial": "17736741",
+          "annulus_count": 83852,
+          "belt_interior_count": 9891,
+          "belt_edge_count": 4022,
+          "table_median_z_m": 0.0020674540221152227,
+          "belt_interior_median_z_m": 0.007149543149646442,
+          "belt_edge_median_z_m": 0.0031961213661946264,
+          "belt_minus_table_m": 0.00508208912753122,
+          "edge_minus_table_m": 0.0011286673440794037,
+          "table_p10_p90_z_m": [
+            0.0009885489451206598,
+            0.0036999330069432796
+          ],
+          "belt_interior_p10_p90_z_m": [
+            0.0028415640039920476,
+            0.021541851704240278
+          ]
+        },
+        {
+          "view": 3,
+          "serial": "23007103",
+          "annulus_count": 55028,
+          "belt_interior_count": 10634,
+          "belt_edge_count": 2844,
+          "table_median_z_m": -0.00021335088196799967,
+          "belt_interior_median_z_m": 0.013696809534714416,
+          "belt_edge_median_z_m": 0.0009112795761381798,
+          "belt_minus_table_m": 0.013910160416682416,
+          "edge_minus_table_m": 0.0011246304581061795,
+          "table_p10_p90_z_m": [
+            -0.001255461659211876,
+            0.0003621480442457137
+          ],
+          "belt_interior_p10_p90_z_m": [
+            0.0026384198928907305,
+            0.024631100914697068
+          ]
+        }
+      ]
+    },
+    "files": {
+      "inputs/depth_offset_diagnostic.json": "be7737ccc32cb91ea97bfcf9a04944fb524fd0f400d8733b99de0ba58bfdbc10"
+    }
+  },
+  "code": {
+    "commit": "d1d2f1381470138dfbb3c58d448e53bfed1e0d7e",
+    "u_pin": "81c718790a37e5e0102eb77af2c6edd34a9db25f"
+  },
+  "wall_time_s": 0.31609106063842773
+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/steps/fit.json b/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/steps/fit.json
new file mode 100644
index 0000000000000000000000000000000000000000..b5e13e925876d87684ba26475c1d2c7873274700
--- /dev/null
+++ b/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/steps/fit.json
@@ -0,0 +1,372 @@
+{
+  "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,
+      "step_t": 0.1,
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+      "u_resolution": 5,
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+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/steps/hybrid.json b/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/steps/hybrid.json
new file mode 100644
index 0000000000000000000000000000000000000000..1f02cc2ffa3ce4e4c522d3ba78b57db6cf7fa3f4
--- /dev/null
+++ b/sessions/session_20260921_232616_062985/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
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+  "outputs": {
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+      "mesh": 21600,
+      "total": 57345,
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+      "hybrid/populations.npz": "b7cd9cef9d316cee4602a99adbf2e123bd5bd6a29487d129fc5c50c769ae89a8"
+    }
+  },
+  "code": {
+    "commit": "d1d2f1381470138dfbb3c58d448e53bfed1e0d7e",
+    "u_pin": "81c718790a37e5e0102eb77af2c6edd34a9db25f"
+  },
+  "wall_time_s": 0.871802806854248
+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/steps/inputs.json b/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/steps/inputs.json
new file mode 100644
index 0000000000000000000000000000000000000000..2adc097a105909bc2a2a9b7340f0a9cf2d4bacf2
--- /dev/null
+++ b/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/steps/inputs.json
@@ -0,0 +1,216 @@
+{
+  "status": "ok",
+  "settings": {
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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_232616_062985/window_anchor0311-0690/accepted_input_preparation_c6a_v1/frames_waft/per_camera_inputs.json",
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+    "unfitted_inputs": true,
+    "fitting_bundle_sha256": "9e139a150804530972c97ab4d3f560e2cf774cc0a9042536debfec6b38d21d20",
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+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/steps/review.json b/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/steps/review.json
new file mode 100644
index 0000000000000000000000000000000000000000..e3d86ae169f5a75e98c96eb616563681b82b3ff2
--- /dev/null
+++ b/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/steps/review.json
@@ -0,0 +1,66 @@
+{
+  "status": "ok",
+  "settings": {
+    "historical_prior": null,
+    "historical_sha256": null,
+    "baseline_fit": null
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+  "outputs": {
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+      "rerun/appearance_spline_prior.rrd": "785d6365e26e90177fb647115416a254bb488960d8cb6c86d9d74786ee9a2dad"
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+}
\ No newline at end of file
diff --git a/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/steps/ribbon.json b/sessions/session_20260921_232616_062985/preprocessing/timing-belt-ribbon-prior-v1/native/steps/ribbon.json
new file mode 100644
index 0000000000000000000000000000000000000000..533deea47dc542bd47637c8d37d450848e31fc64
--- /dev/null
+++ b/sessions/session_20260921_232616_062985/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": {
+    "checks": {
+      "representation": "zero-thickness intrinsic ribbon surface (open width edges)",
+      "euler_characteristic": 0,
+      "watertight": false,
+      "components": 1,
+      "winding_consistent": true,
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+      "confirmed_intersecting_pairs": [],
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+      "u_resolution": 5,
+      "passed": true
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+    "triangle_count": 720,
+    "files": {
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+      "ribbon/ribbon_checks.json": "2dc305fe10f131dfef31abcd99af9dd7144a5c2018577e40df27f4109d73b964"
+    }
+  },
+  "code": {
+    "commit": "d1d2f1381470138dfbb3c58d448e53bfed1e0d7e",
+    "u_pin": "81c718790a37e5e0102eb77af2c6edd34a9db25f"
+  },
+  "wall_time_s": 3.4238064289093018
+}
\ No newline at end of file