Sprite tool: drop sky stuck to the subject, build the sprite in a worker

[Finesssee]

Viewers who turn a photo into a sprite get two fixes.

• Sky stuck to the subject. A cloud touching the top or side edge, and matching patches cut off beside the head, stayed in the sprite (the lake duck). dropEdgeLeak in src/autosprite.ts removes them. It backs off when that would remove more than 35% of the subject, so a duck on ice keeps its ice. Bench masks on the 9 test pictures match the approved run.
• Page freeze. Building the sprite froze the fighter page for 0.3 to 1.3 s. It now runs in a Web Worker (src/sprite-worker.ts); a newer build replaces an older one. CSP needs no change (script-src 'self' covers a same-origin worker).

Tests: a unit test for the leak removal in tests/spritify.test.ts (red with either pass disabled). bun run check 392 of 392, tests/ui.ts and tests/arena-browser.ts pass locally. Docs: docs/CONTRACTS.md viewer sprites.

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