docs(examples): coincident-vert-weld README states who hits it and what catches it, re-verified on 5.1

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

Author: TMHSDigital

examples/coincident-vert-weld/README.md

@@ -8,23 +8,62 @@ Inverse of [`degenerate-bevel-weld`](../degenerate-bevel-weld/)
 solid). Neighbor of [`gltf-export-roundtrip`](../gltf-export-roundtrip/)
 (kit-bash face-plane welds on export).
 
-**Why this pathology:** coincident shells are still manifold (every
-edge borders 2 faces). Hygiene can pass. The engine trap is extra
-triangles on disk.
+## The contract
+
+A mesh holding two coincident shells is still **manifold**: every edge
+borders exactly two faces, because each shell is closed on its own. A
+hygiene check that only counts non-manifold edges reports 0 and passes.
+The glTF exporter then writes both shells: nothing in
+`bpy.ops.export_scene.gltf` welds coincident vertices, so the file
+carries twice the triangles of the visible solid.
+
+**Who hits this:** kit-bash and boolean-cleanup pipelines, duplicated
+objects joined back onto themselves, and generators that emit a part
+twice. The asset looks like one cube in Blender and in the engine: the
+two shells are identical triangles, so the depth test hides the second
+without flicker. The cost is invisible in the viewport and real on disk
+and at runtime: double the draw triangles, a collider built from the mesh
+twice as heavy, and overlapping lightmap UVs if the shells were unwrapped.
+
+**What catches it:** a coincident-vertex check, not a manifold check.
+`bmesh.ops.find_doubles(dist=1e-5)` reports **8** doubles on this mesh
+while the non-manifold count stays **0**. That is why the showcase
+hygiene budgets count doubles separately from non-manifold edges. The
+fix is `bmesh.ops.remove_doubles` before export.
+
+## What the check asserts
 
 **Pre-assertion (pathology exists):** **16** verts, **8** unique
-positions, **12** faces, **24** edges, valence all 2. `--no-duplicate`
-exits 3.
+positions, **12** faces, **24** edges, valence 2 on every edge.
+`--no-duplicate` builds a single cube and exits 3. Vert count 16 alone is
+not enough, since any 16-vert mesh has it. Unique positions = 8 is the
+construction witness.
+
+**Handling (second axis):** read back from the written `.gltf` and its
+`.bin`: **48** loop-split positions, **24** triangles, **8** unique
+positions. `--weld` runs `remove_doubles` after the pre-assertion, the
+file then carries 24 positions and 12 triangles, and the check exits 4.
+
+`remove_doubles` collapses the pair to one cube: 8 verts, 12 edges,
+6 faces, valence 2. That is identical in every count to a cube built
+once. It does not leave a 4-faces-per-edge mesh.
+
+## Versions
+
+Re-verified on Blender 4.5.11 LTS, 5.1.2 and 5.2.1 LTS. Every count
+above is the same on all three, including the `find_doubles` and
+non-manifold figures and both falsifier exits. Not a version split.
 
-**Handling (second axis):** glTF ships **48** loop-split positions,
-**24** tris, **8** unique. Vert count 16 is not enough; unique=8 is
-the construction axis; 24 tris vs 12 after `remove_doubles` is the
-export axis. `--weld` after the pre-assert exits 4.
+No gallery still. Two coincident cubes look like one cube; the defect
+is only in the counts.
 
-`remove_doubles` collapses to one cube (8/12/6), still manifold — not
-a 4-face-per-edge mesh. Same on 4.5 LTS and 5.2 LTS.
+## API reference
 
-No gallery still. Two coincident cubes look like one cube.
+- [`bmesh.ops.find_doubles`](https://docs.blender.org/api/current/bmesh.ops.html#bmesh.ops.find_doubles)
+  and [`bmesh.ops.remove_doubles`](https://docs.blender.org/api/current/bmesh.ops.html#bmesh.ops.remove_doubles)
+  ([4.5 LTS](https://docs.blender.org/api/4.5/bmesh.ops.html#bmesh.ops.remove_doubles))
+- [`bpy.ops.export_scene.gltf`](https://docs.blender.org/api/current/bpy.ops.export_scene.html#bpy.ops.export_scene.gltf)
+  ([4.5 LTS](https://docs.blender.org/api/4.5/bpy.ops.export_scene.html#bpy.ops.export_scene.gltf))
 
 ## Run