feat: point lights (direction-only shading + radial colored shadows)

[apresmoi]

Adds point lights to PolyCSS across core, vanilla, React, and Vue, and overhauls the shadow color model.

Lights

• New PolyPointLight (position, color, intensity, castShadow). Direction-only — no distance falloff — emulating three.js PointLight(distance:0, decay:0). Flat-per-face Lambert; accumulates per-channel with the directional + ambient terms.
• Point lights are baked-mode only (dynamic mode's zero-JS light move can't express a per-face direction); dynamic scenes ignore them for both shading and shadows.
• pointLights scene option mirrored across all renderers (per-mesh local conversion + bake/re-bake wiring).

Shadows

• Radial point-light cast shadows: each vertex projects along its own ray from the light; point passes project the caster silhouette (per-face back-faces leave the contact footprint unshadowed under radial divergence).
• Shaded, not flat-black: each light's shadow shows the receiver lit by every other light, so a region blocked from one colored light keeps the others' color (Three.js colored shadows). A lone directional light reduces to the ambient-only fill (unchanged).
• Correct overlap: all of a receiver face's lights merge into one SVG — multi-light solid faces paint a full-lit base + per-light mix-blend-mode: multiply layers, so the both-blocked region composites to ambient-only. Shared via core computeMergedReceiverShadows so vanilla/React/Vue are identical.
• No directional shadow at zero/absent intensity (Three.js parity); dropped the implicit-sun fallback. setOptions now re-emits shadows on directional intensity/color change, not just direction.

Docs & bench

• New Lighting & Shadows guide with an interactive lit-cube demo (light direction/intensity/ambient controls + cast shadow); PolyDemo gains light controls and an opt-in ground shadow.
• Documented the baked rebake contract (vanilla explicit rebakeAtlas() vs React/Vue auto-rebake) and the dynamic point-light exclusion.
• Bench: point-light oracle (vs three.js PointLight(decay=0)), shadow-color delta tool, and a 4-pane vanilla/React/Vue/three parity page.

All packages green (core 1087, polycss 747, react 435, vue 434, fonts 45) and pnpm build clean.