feat(rendering): add nine-slice support for sprites

[stormmuller]

Summary

Closes #471.

• Adds NineSliceOptions (left/right/top/bottom border insets, plus independent edgeMode/centerMode of 'stretch' (default) or 'tile') as an optional slices field on SpriteEcsComponent, and as an option on createImageSprite.
• computeNineSliceRegions is a new pure utility that turns a sprite's current size/pivot/UV rect + NineSliceOptions into up to nine region descriptors (four fixed-size corners, four stretch/tile edges, one stretch/tile center), correctly clamping oversized borders and anchoring UV fractions to a nativeWidth/nativeHeight reference so resizing a sliced sprite never distorts its corners.
• createRenderEcsSystem expands a sliced sprite into one RenderCommand per region (still batched into a single instanced draw call alongside every other sprite sharing the same Renderable) instead of writing a new shader/geometry path — the existing generic sprite shader already supports arbitrary per-instance size/position/UV, so no shader or geometry changes were needed. Region offsets correctly compose with the entity's rotation, scale, and flip.
• 'tile' uses "round" repeat semantics (evenly-sized tiles, no cropped partial tile at the seam), the same tradeoff as CSS's border-image-repeat: round.

Note on SpriteOptions

The issue links to the SpriteOptions API page (src/rendering/sprite.ts's Sprite class). That class predates the ECS component-factory refactor and has zero references anywhere in src, the demos, or the docs — the actual render pipeline (createRenderEcsSystem) only ever reads SpriteEcsComponent. This PR adds slicing to the live SpriteEcsComponent/createImageSprite path instead of extending the dead Sprite class.

Test plan

• npm run check-types
• npm test (989 tests, including new unit tests for computeNineSliceRegions and new createRenderEcsSystem nine-slice cases)
• npm run lint
• npm run cspell
• npm run check-exports
• Added a "Nine-Slice Sprites" doc page under documentation-site/docs/docs/rendering
• documentation-site: npm run build + npm run typecheck pass; loaded the ecs and stress-test demos in a browser to confirm no rendering regression (both demos don't use slicing yet, but exercise the same createRenderEcsSystem code path)

---

Generated by Claude Code