feat: add grease-pencil-rosette example (GPv3 attribute API drift witness)

[TMHSDigital]

Summary

Adds examples/grease-pencil-rosette — five nested neon rose curves drawn entirely with the Grease Pencil v3 attribute-based API (layer → frames.new(1).drawing → add_strokes([counts]) → per-point position/radius/opacity/vertex_color). GPv3 is the largest bpy API break in the 4.x-to-5.x window and nothing in the gallery covered it; this is the strongest drift witness the gallery can hold.

API contract witnessed

• The address break (version-gated, asserted on each side):
  • 4.5 LTS: GPv3 is bpy.data.grease_pencils_v3 (GreasePencilv3); bpy.data.grease_pencils still holds legacy GPencil whose frames carry .strokes and have no .drawing — the trap is asserted directly against a scratch legacy datablock, not just avoided.
  • 5.x: legacy is deleted; GPv3 owns the grease_pencils name; grease_pencils_v3 and bpy.types.GPencilStroke no longer exist.
• Lazy attribute materialization: writing pt.radius/pt.opacity/pt.vertex_color through the point view creates radius, opacity, vertex_color (POINT) and cyclic (CURVE) layers in drawing.attributes with the expected domains/types.
• Points are views over attributes: all 5×192 closed-form rose positions round-trip through the raw position POINT buffer via foreach_get (worst deviation observed: 5.9e-08).
• Object type is GREASEPENCIL; bpy.data.materials.create_gpencil_data works identically on both versions.

Visual notes

Neon-sign composition: unlit (use_lights = False) tapered strokes, per-ring hue sweep cyan→violet→magenta→red, Standard view transform (AgX desaturates unlit saturated ink), a single area light painting a soft halo on the wall behind the rosette, floor horizon for grounding. Inspected at full 1280×720 and at thumbnail scale; nothing clipped, no visible lights, no blown highlights.

Integration

gallery.json entry, .cursor-plugin/plugin.json examples array, blender-smoke.yml step, README gallery row + counts (16→17), AGENTS.md/CLAUDE.md counts, ROADMAP candidate-pool follow-ups, regenerated docs/gallery/ via scripts/build_gallery.py. No release-owned version fields touched.

Test plan

Proven by live run (local):

• Check-only path on Blender 5.1.2 — exit 0
• Check-only path on Blender 4.5.11 LTS — exit 0
• --output render on 5.1.2 (EEVEE) — hero/preview webp generated from the inspected render
• python scripts/build_gallery.py — 17 detail pages, only expected files changed
• JSON validity of gallery.json and plugin.json

Established by inspection only:

• Manifest/count CI jobs (validate-counts, validate-manifest) — wired per convention, verified against the greps they use, but exercised only by this PR's CI
• 4.5 render path (--engine cycles fallback) — code path mirrors bmesh-gear; check-only was live-run on 4.5

🤖 Generated with Claude Code