feat: add armature-bend example (edit_bones + LBS closed-form witness)

[TMHSDigital]

Summary

Adds examples/armature-bend — the highest-priority gallery coverage gap (rigging): a four-bone armature chain built with edit_bones skins a tapered, segmented tube through name-bound vertex groups with smoothstep blend zones, poses a curl, and reads the result back through the depsgraph.

API contract witnessed

1. edit_bones lifetime — asserted empty in object mode; inside mode_set(mode='EDIT') the chain must exist with heads/tails at closed-form positions. Reads outside edit mode silently return nothing.
2. Vertex groups bind by name — every group is named after its bone; a typo deforms nothing without erroring. The deform proves the binding.
3. The armature modifier is exactly linear blend skinning — every depsgraph-evaluated vertex is compared against a re-implemented Σ wᵢ · (pose_bone.matrix @ bone.matrix_local.inverted()) @ rest (max error observed 4.77e-07); the root ring must stay pinned and the tip must deflect. A straight tube exits non-zero.

4.5/5.1 divergence: the armature API itself is identical on both — no version gate. A real portability hazard is documented in-code: holding a color_attributes reference across VertexGroup.add() dangles it when the deform CustomData layer allocates — a hard EXCEPTION_ACCESS_VIOLATION on 4.5.11 that 5.1.2 survives by luck. The script orders all weight writes before creating the attribute.

Visual notes

Rest → half curl → full curl left to right; the rigs are rotated so the curl sweeps across the picture plane. Per-bone weight bands (teal → cyan → amber → coral) are a BoneTint color attribute computed from the same weights the LBS check asserts, so the color blends at the joints are the contract. Dark-studio recipe matching the existing gallery. Inspected at full 1280×720 and at 1200×675 preview scale.

Test plan

Proven by live run (local):

• Check-only path on Blender 4.5.11 LTS — exit 0, lbs_max_err=4.77e-07 tip_deflection=3.191
• Check-only path on Blender 5.1.2 — exit 0, identical figures
• --output render on 5.1.2 (EEVEE), pixels inspected at full size and thumbnail scale
• python scripts/build_gallery.py regenerated (18 detail pages)
• Local equivalents of validate-counts (12/6/2/17/18) and validate-manifest (all paths exist, examples list matches disk, version == VERSION)

Established by inspection only:

• Generated gallery HTML diff limited to the new entry
• CI smoke jobs (Linux/xvfb) — this PR's checks are the authoritative run

🤖 Generated with Claude Code