feat: add parent-inverse-orrery example (matrix_parent_inverse contract)

[TMHSDigital]

Summary

Adds examples/parent-inverse-orrery/ — the sixteenth smoke-gated example. A brass tabletop orrery (sun, three planets on pivot arms inside orbit rings, one moon) assembled entirely through bpy.data, where the object hierarchy itself is the subject: every arm, planet, and the two-level moon assembly is parented with the keep-world idiom and then spun to its display angle by rotating only the pivots.

No existing example covers object parenting or the transform-update contract, and both are things AI-generated Blender code reliably gets wrong.

API contract witnessed

1. The trap is real — probe.parent = pivot alone re-interprets the probe's local matrix in the pivot's space: the check asserts the probe jumps (> 0.5 units).
2. The fix is exact — probe.matrix_parent_inverse = pivot.matrix_world.inverted() restores the world position to within 1e-5.
3. matrix_world is the last-evaluated matrix — after a transform edit it must stay stale until bpy.context.view_layer.update(), then reflect the edit.
4. Closed-form orbits — every planet lands exactly on Rz(theta) @ p0, and the moon on the two-level composition pc + Rz(theta1 + thetam) @ (m0 - pc0), proving the parenting algebra carries a real hierarchy.

Exits non-zero (codes 3–7) on any violation. Check-only by default; --output renders the gallery still (EEVEE, --engine cycles for GPU-less hosts).

Visual notes

Dark-studio recipe from bmesh-gear: key/fill/rim area lights, floor + wall, faint warm world. Brass column, arms, and POLY-circle orbit rings; emissive sun crowning the column; lapis/terracotta/jade planets on three ring levels; silver moon on its rod beside the outer planet. Verified at full size and thumbnail scale — subject silhouette and hierarchy read clearly in both.

Test plan

• blender --background --python examples/parent-inverse-orrery/parent_inverse_orrery.py -- exits 0 on Blender 5.1.2 (local)
• Same check-only run exits 0 on Blender 4.5.11 LTS (local)
• -- --output orrery.png renders on both 5.1.2 and 4.5.11; final image visually inspected at 1280×720 and thumbnail size
• python scripts/build_gallery.py regenerated docs/gallery/ (index + 16 detail pages)
• gallery.json, plugin manifest, smoke workflow step, README row + counts, AGENTS.md/CLAUDE.md counts all updated
• CI: validate (structure/counts/manifest), drift-check, Blender 4.5 + 5.1 smoke jobs green

🤖 Generated with Claude Code