feat: add uv-layer-grid example (create_grid calc_uvs silent no-op)

[TMHSDigital]

Summary

• Ships examples/uv-layer-grid/ witnessing that bmesh.ops.create_grid(..., calc_uvs=True) is a silent no-op unless a UV layer already exists — the hazard that makes Image Textures sample texel (0,0) everywhere.
• Checks: hazard (0 layers), pre-create + calc_uvs against closed-form UVs ((x/size+1)/2, (y/size+1)/2) within 1e-6 (bmesh + mesh persistence), and an explicit loop-assignment fallback. Dual-panel gallery still: flat teal (broken) beside neon checker (repaired).
• Wired gallery.json, plugin manifest, blender-smoke, README counts (21), ROADMAP (shipped + added attribute-domain candidate).

API contract / falsification

Check	Failure caught	Falsified
Silent no-op	calc_uvs=True starts creating a layer alone	Pre-created a layer before the hazard probe → exit 3
Closed-form UVs	Wrong parameterization / shift	Added 0.1 to every U after calc_uvs → exit 6, measured err 1.00e-01

Test plan

• Check-only on Blender 5.1.1 locally — measured: 0 UV layers on hazard path; calc/mesh/explicit errs 0.00e+00 (tol 1e-06)
• Check-only on Blender 4.5 LTS — not installed locally (host has 4.4.3 + 5.1.1 only); rely on blender-smoke 4.5 job
• Cycles gallery render + visual inspect (full + thumbnail); webp hero/preview generated
• python scripts/build_gallery.py; local count/manifest/gallery path checks
• CI: validate, drift, blender-smoke 4.5 + 5.1

Visual notes

Two easel panels, dark studio + warm wedge. Left = missing UV (flat teal of texel 0,0). Right = pre-create + calc_uvs (magenta/cyan checker). Failure of the UV contract would make both panels identical.