fix(examples): redesign gltf-skin-roundtrip as a mech scorpion standoff

The review singled this hero out: two near-identical scorpions built from
untextured orange boxes, with legs clipping into each other and the floor.
The image gave the eye nothing to compare, because a pass meant "they look
the same".

The model is rebuilt from scratch. It has a lofted gunmetal chassis under a
crowned carapace and five shingled tergite plates. Eight jointed legs have
painted femur armour, hydraulic rams and knee drums, and they plant on
rubber pads. Each claw has three bones (arm, swollen chela, movable finger).
The tail has five armoured collars on rubber bellows, and a telson with a
venom band and a chrome needle. There are 5 materials, and emission is used
only on the eyes and the venom band. The rig grows from 7 bones to 21, with
bones pointing +Y, -Y and +/-X, so the rest-matrix round-trip is exercised
off the old single axis.

The composition now witnesses the skin. The authored rig holds the guard
pose the check compares. The actual re-imported mesh is driven through its
IMPORTED armature into a different strike pose. A file that lost the skin,
the joints or the weights could not follow that pose. The render path now
runs gallery_framing (exit 10) and gallery_asset_quality (exit 11).

Two check-protocol bugs were found and fixed; both make the check stricter:
- The "straddle-safe" neighbour lookup added +/-1e-4 without re-rounding,
  so only the exact key could ever match. It reported a phantom weight
  loss (exit 18, 1.0) once the redesign put a vertex on a rounding
  boundary.
- got_w kept only the last re-imported vertex per position. A corrupted
  split copy hid behind its twin, and the weight falsifier fell through to
  exit 19 instead of 18.

Signed-off-by: TMHSDigital <154358121+TMHSDigital@users.noreply.github.com>
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: TMHSDigital

README.md

@@ -933,13 +933,13 @@ hazard that silently corrupts measurements on 4.5.
 </tr>
 <tr>
 <td width="46%" valign="middle">
-<a href="examples/gltf-skin-roundtrip/"><img src="examples/gltf-skin-roundtrip/preview.webp" alt="glTF skin round-trip: two identical mech scorpions on a dark studio floor - orange armor, black segmented tails curled over their backs with glowing stinger tips, teal eyes - proving skeleton, weights, and deformation survive the export" /></a>
+<a href="examples/gltf-skin-roundtrip/"><img src="examples/gltf-skin-roundtrip/preview.webp" alt="glTF skin round-trip: two orange-plated mech scorpions face off on a dark studio floor - the authored one on the left with its tail coiled over its back and claws tucked, the re-imported one on the right driven through its imported bones into a strike, tail reared high and claws raised" /></a>
 </td>
 <td valign="middle">
 
 ### [gltf-skin-roundtrip](examples/gltf-skin-roundtrip/)
 
-The skinning counterpart to `gltf-export-roundtrip`: a seven-bone rigged
+The skinning counterpart to `gltf-export-roundtrip`: a 21-bone rigged mech
 scorpion exported with `export_skins` and re-imported, asserting the joint
 list, JOINTS_0/WEIGHTS_0 unit sums, bone/parent/rest-matrix round-trip,
 bit-exact weights, and identical deformation of the re-imported rig — plus

docs/gallery/asset-sheets/gltf-skin-roundtrip.webp

@@ -662,12 +662,12 @@ <h2><a href="triangulate-tangents/">triangulate-tangents</a></h2>
       </article>
       <article class="card" data-tags="export armature">
         <a class="card-media" href="gltf-skin-roundtrip/" tabindex="-1" aria-hidden="true">
-          <img src="assets/gltf-skin-roundtrip-hero.webp" alt="Two blocky orange scorpion robots with curled tails and glowing cyan stingers, authored and re-imported." loading="lazy" decoding="async" />
+          <img src="assets/gltf-skin-roundtrip-hero.webp" alt="Two orange-plated mech scorpions face off: the authored one coils its tail with claws tucked; the re-imported one, driven by its imported bones, rears its tail to strike with claws raised." loading="lazy" decoding="async" />
         </a>
         <div class="card-body">
           <h2><a href="gltf-skin-roundtrip/">gltf-skin-roundtrip</a></h2>
           <p class="teaches">A rigged mech scorpion exported to glTF with skins and re-imported, verifying the skinning contract the geometry round-trip left uncovered. Skeleton, weights, and deformation must all survive the format.</p>
-          <p class="witnesses"><span class="tag">witnesses</span> skins[0].joints names every bone; JOINTS_0/WEIGHTS_0 per primitive with unit sums; weights bit-exact on re-import; deformation matches within 4.8e-07. The mesh must be parented to the armature or the exporter binds by name.</p>
+          <p class="witnesses"><span class="tag">witnesses</span> skins[0].joints names all 21 bones; JOINTS_0/WEIGHTS_0 per primitive with unit sums; every split copy&#x27;s weights bit-exact on re-import; deformation matches within 6.0e-07. The mesh must be parented to the armature or the exporter binds by name.</p>
           <a class="card-link" href="gltf-skin-roundtrip/">View example<span class="sr-only"> gltf-skin-roundtrip</span> <span aria-hidden="true">&rarr;</span></a>
         </div>
       </article>

docs/gallery/assets/gltf-skin-roundtrip-hero.webp

@@ -393,8 +393,8 @@
       "name": "gltf-skin-roundtrip",
       "dir": "examples/gltf-skin-roundtrip",
       "teaches": "A rigged mech scorpion exported to glTF with skins and re-imported, verifying the skinning contract the geometry round-trip left uncovered. Skeleton, weights, and deformation must all survive the format.",
-      "alt": "Two blocky orange scorpion robots with curled tails and glowing cyan stingers, authored and re-imported.",
-      "witnessesFix": "skins[0].joints names every bone; JOINTS_0/WEIGHTS_0 per primitive with unit sums; weights bit-exact on re-import; deformation matches within 4.8e-07. The mesh must be parented to the armature or the exporter binds by name.",
+      "alt": "Two orange-plated mech scorpions face off: the authored one coils its tail with claws tucked; the re-imported one, driven by its imported bones, rears its tail to strike with claws raised.",
+      "witnessesFix": "skins[0].joints names all 21 bones; JOINTS_0/WEIGHTS_0 per primitive with unit sums; every split copy's weights bit-exact on re-import; deformation matches within 6.0e-07. The mesh must be parented to the armature or the exporter binds by name.",
       "hero": "docs/gallery/assets/gltf-skin-roundtrip-hero.webp",
       "preview": "examples/gltf-skin-roundtrip/preview.webp",
       "tags": [

docs/gallery/contact-sheets/gltf-skin-roundtrip-contact-sheet.webp

@@ -1,7 +1,9 @@
 # glTF Skin Round-Trip
 
-A runnable example that rigs a mech scorpion — seven-bone chain from pedestal
-root to stinger, blend rings at every joint seam — exports it with
+A runnable example that rigs a mech scorpion — 21 bones: a body, a
+five-segment tail and stinger, two three-bone claws (arm, hand, movable
+finger), eight legs; two-bone blends in the rubber boots at every flexing
+joint — exports it with
 `bpy.ops.export_scene.gltf` (`export_skins=True`), parses the file, re-imports
 it, and verifies the whole skinning contract against the authored rig,
 following [`depsgraph-and-evaluated-data`](../../skills/depsgraph-and-evaluated-data/SKILL.md).
@@ -16,18 +18,22 @@ maps are in [`triangulate-tangents`](../triangulate-tangents/).
 round-trip left uncovered.
 
 - **The skeleton survives.** `skins[0].joints` names every bone; the
-  re-imported armature carries the same 7 bones, the same parent chain, and
-  rest matrices within 2.4e-07 — the +Y-up conversion applies to bone nodes
-  exactly as it does to meshes (their translations convert, no rotation is
-  written).
+  re-imported armature carries the same 21 bones, the same parent chain,
+  and rest matrices within 2.3e-06 — the +Y-up conversion applies to bone
+  nodes exactly as it does to meshes, whichever way a bone points (tail
+  +Y, claws -Y, legs +/-X).
 - **The weights survive.** Every primitive carries JOINTS_0/WEIGHTS_0;
-  per-vertex weights on disk sum to 1 (err 3.0e-08); the re-imported vertex
-  groups match the authored groups **bit-exactly** (w_err 0.0), compared as
-  straddle-safe position keys, the same protocol as the crate example.
+  per-vertex weights on disk sum to 1 (err 3.0e-08; 840 disk vertices carry
+  two influences); the re-imported vertex groups match the authored groups
+  **bit-exactly** (w_err 0.0), compared as straddle-safe position keys, the
+  same protocol as the crate example. Every split copy at a position is
+  compared, not just one: the exporter splits vertices per normal, and each
+  copy carries its own JOINTS_0/WEIGHTS_0, so one corrupted copy must not
+  hide behind a good twin.
 - **The deformation survives.** Posed identically, the re-imported rig's
-  evaluated mesh matches the original's within 4.8e-07 — linear blend
+  evaluated mesh matches the original's within 6.0e-07 — linear blend
   skinning through the file format. The comparison is by rest-position key,
-  never sorted multisets: the exporter welds duplicate loops (32 here), so
+  never sorted multisets: the exporter welds duplicate loops (9960 here), so
   cardinalities differ and a naive sorted zip mispairs vertices (a phantom
   2.29 "deviation" measured and fixed during authoring).
 - **The mesh must be parented to the armature.** The exporter warns
@@ -36,21 +42,40 @@ round-trip left uncovered.
 
 **What each check catches on failure:** exporting with `export_skins=False`
 (exit 5 — no skin on disk), stripping the weights (exit 4 — no vertex
-groups), and posing the re-imported rig differently (exit 19 — deformation
-deviates 0.90).
-
-**Version witness:** the skins pipeline is stable between Blender 4.5 LTS and
-5.1 — the exporter/importer RNA is byte-identical (probed with
-`gltf-export-roundtrip`), and every measured value matches to the digit on
-4.5.11 and 5.1.2.
-
-The render stages the authored scorpion beside the actual re-imported one —
-same curl, same glowing stinger — proof the skin rode the format through.
-Both rigs are turned three-quarters to the camera. The tail curls in the
-rig's own Y-Z plane, so a head-on view saw the curl end-on: the tail stood
-straight up behind the body like a chimney. The yaw is composed into each
-armature's world matrix, because the glTF importer leaves its armature in
-quaternion mode, where `rotation_euler` does nothing.
+groups), nudging one re-imported weight (exit 18 — 0.99), moving one
+re-imported bone's rest tail 1 mm (exit 14 — 3.9e-03), and posing the
+re-imported rig differently (exit 19 — deformation deviates 0.13).
+
+**Straddle-safe keys, for real.** A float32 round-trip can land a
+coordinate on the far side of a 1e-4 rounding boundary, so lookups also try
+the 26 neighbouring keys. Those neighbours must themselves be rounded:
+`0.3601 + 1e-4` in binary floating point is not the key `0.3602`. The first
+version of this example added the offset without rounding, so only the
+exact key ever matched. It passed by luck until the redesign lowered the
+body to z = 0.36, and then it reported a phantom weight loss (exit 18, 1.0).
+
+**Version witness:** the skins pipeline is stable across Blender 4.5 LTS,
+5.1 and 5.2 LTS — every measured value matches to the digit on 4.5.11,
+5.1.2 and 5.2.1.
+
+The render is a standoff. On the left, the authored scorpion holds the
+guard pose the check compares: tail coiled tight over its back, claws
+tucked, pincers shut. On the right is the actual re-imported mesh, driven
+through its **imported** armature into a different pose: the tail rears
+high to strike, the claws are raised and the pincers gape. A file that lost
+the skin, the joints, or the weights could not follow that pose. The
+rubber boots at the joints bend smoothly and the plates stay rigid, which
+is the linear blend skinning surviving the format. The two poses differ on
+purpose, so the eye has something to compare. Two identical copies would
+render the same whether or not the round-trip worked.
+
+Both rigs are posed by rest-space axes in quaternions, and the yaw is
+composed into each armature's world matrix, because the glTF importer
+leaves its armature in quaternion mode, where `rotation_euler` does
+nothing. Framing is measured by `gallery_framing` (fill 0.856 x, minimum
+margin 0.072). The asset floors are measured by `gallery_asset_quality`
+(5 materials: hazard paint, gunmetal, chrome, rubber, venom glow; edge90
+0.001).
 
 ## Run
 
@@ -95,6 +120,11 @@ against it.
 | 19 | Deformation round-trip drifted |
 | 20 | `--output` produced no file |
 
+On the `--output` path only, which runs after every check has passed, 10
+also means a `gallery_framing` violation and 11 a `gallery_asset_quality`
+floor violation. The shared helpers own those codes, and the log line says
+which one fired.
+
 The `blender-smoke` workflow runs the check on Blender 5.2 LTS and 4.5 LTS
 (5.1 on the weekly cron, the `needs-5.1` PR label, or manual dispatch).
 Smoke does not pass `--output` or `--no-skins`.