fix(shadow): kill cast/self-shadow holes on imported meshes + shadow-parity bench tooling

[apresmoi]

Summary

Fixes a family of cast-shadow and self-shadow holes on imported (non-watertight / faceted) meshes, and adds shadow-debugging tooling to the bench. All shadow fixes live in packages/core (shared by vanilla/React/Vue) plus the vanilla/React/Vue caster glue; the tooling is bench-only.

The work was driven by a new shadow-mask parity oracle (below): it reduces both engines to a binary "in shadow?" mask and reports a single shadowIoU per view, so each fix was validated against three.js as ground truth rather than by eyeballing.

Shadow fixes (packages/core + renderer glue)

1. Silhouette-reliability gate. The H9 silhouette fast path mis-chains loops at degree-≠2 vertices (non-manifold / T-junctions / open boundaries from import), leaving gaps. Now it's used only when the silhouette is a clean union of simple closed loops; otherwise it falls back to the gap-free per-polygon union.
2. Cast from all polygons. Shadow casters were filtered to camera-rendered polygons (atlas plan) and de-duplicated — so a polygon facing the light but not the camera vanished, producing camera-dependent floor-shadow holes. Now every polygon casts; coincident projections merge under the per-mesh fill-rule: nonzero.
3. Double-sided casting for non-watertight + self-shadow. Single-sided casting dropped light-back-facing occluders. The per-poly path now casts double-sided for unreliable cross-mesh casters AND all self-shadow casters (e.g. flight poly 46 recovered 353 dropped occluders). Closed meshes are unaffected — their far back-faces sit below each lit receiver plane and get above-plane-culled (verified: apple unchanged).
4. Gated self-shadow seam cull. The seam cull (for smooth-mesh "spiderweb" slivers) was too blunt and deleted real self-shadows on faceted meshes. It now fires only when the caster is near-coplanar (~20°) with the receiver face.

Measured (shadow-mask oracle, IoU = shape parity, 1.0 = perfect)

view	before → after
rotated/sunk castle floor	IoU 0.978 → 0.992 (missing 2291 → 717 px)
flight self-shadow (seam)	IoU 0.866 → 0.951 (missing 5775 → 1221 px)
apple (smooth, regression check)	unchanged (no spiderweb)

Bench tooling (bench/)

• Real-capture shadow oracle (shadow-oracle.html): captures the real compositor (Playwright screenshot / tab-capture) instead of html-to-image, which silently dropped ~99.5% of the 3D-transformed shadow SVGs. Diffs across all meshes with per-polygon attribution.
• Shadow-mask parity oracle: binary shadow-shape diff with tolerance dilation → shadowIoU, missing/extra pixel counts, rotation-correct occluder attribution. Headless drivers: bench/scripts/shadow-mask-verdict.mjs, shadow-oracle-verdict.mjs.
• Light-animation panel: drive the directional light with per-axis math expressions of a looping phase t∈[0,2π) (sin/cos/…), with play/pause, speed, and scrub.

Tests & build

pnpm test green (core 1075, polycss 747, react 435, vue 434, fonts 45) and pnpm build (packages + website) green. AGENTS.md updated to reflect the new shadow caster rules.