fix(rendering): make SpriteEcsComponent.pivot Y-up to match the rest of the engine

[stormmuller]

Summary

SpriteEcsComponent.pivot used a Y-down convention ((0, 0) was the sprite's top-left) while world position, rotation, and every other Y-facing value in the engine are Y-up. The mismatch was an internal projection detail (the vertex shader flips Y before the projection matrix flips it back for position/rotation) leaking through pivot's public API - invisible at the default centered pivot, which is why it survived.

Traced the full pipeline per the issue and confirmed the analysis: bindSpriteInstanceData pre-negates position.world.y and rotation.world to compensate for the projection's Y flip, but sprite.vert.glsl computes the pivot-based local offset after that compensation, so it never gets flipped - pivot.y = 0 landed at the sprite's top instead of its bottom.

Changes

• sprite.vert.glsl: negate a_instancePivot.y alongside the existing [0,1] -> [-1,1] doubling, so pivot's Y now gets the same flip position/rotation already get. (0, 0) is now bottom-left, (1, 1) is top-right.
• computeNineSliceRegions (compute-nine-slice-regions.ts): updated the region-offset math ((1 - pivot.y) * height - ... instead of pivot.y * height - ...) so nine-slice region placement agrees with the corrected shader convention. The centered pivot (0.5, 0.5) produces identical output before and after (1 - 0.5 === 0.5), so this is a no-op for every sprite in the repo (no demo or test uses a non-centered pivot).
• sprite-component.ts: updated pivot's JSDoc to describe the new Y-up convention.
• compute-nine-slice-regions.test.ts: replaced the single top-left-pivot test with a parameterized case pinning all four corners (bottom-left, bottom-right, top-left, top-right), per the issue's test-coverage callout.
• CHANGELOG.md: added a Fixed entry under [Unreleased], flagged as a breaking change for non-centered pivots.

Related issue(s)

Closes #585

Out of scope

The issue's design doc reference (design/ui-system.md's pivot bridge, from #580) isn't in dev yet, so there's nothing to remove there in this PR.

Verification checklist

• npm run check-types passes with 0 errors
• npm test passes (1036 tests)
• npm run lint passes with 0 errors (pre-existing, unrelated TODO warnings only)
• npm run cspell passes with 0 errors
• npm run check-exports passes
• No public API surface changed beyond pivot's documented semantics - no export changes needed
• Documentation under /documentation-site/docs/docs - no page documents the pivot convention, so nothing to update
• No demo uses a non-centered pivot (verified via grep), so no demo code changes; still built /dist, ran documentation-site's typecheck/build, and visually checked the nine-slice and brick-breaker demos in a browser to confirm no regression at the default centered pivot

Changelog

• Added a bullet under ## [Unreleased] → #### Fixed in CHANGELOG.md

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[stormmuller]

In the ECS demo. The position looks correct, but the rotation looks inverted?