FontAtlas::Update used a single union bounding box for the dirty rect.
Scattered new glyphs landing on different shelves produced a tall union
box that re-uploaded mostly-unchanged texels between the shelves.
Track a dirty span per shelf instead and issue one tight UpdateRegion /
wgpuWriteAtlasRegion copy per armed span. A shelf packs glyphs
left-to-right at a fixed top, so each span stays a contiguous X run
capped by the shelf height. The whole-atlas zero-clear in Initialize
keeps using the existing dirtyRect span. `dirty` is now the OR of every
span and is cleared together with them in Update.
Verified: full test suite green (23 passed); HelloUI text renders
crisply on the WebGPU backend through the new multi-span upload path.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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