fix: cancel scroll completion checks on unmount

[JacquesLeupin]

Summary

• cancel the stored scroll-completion animation frame and fallback timeout when LegendList unmounts
• cancel a pending completion frame before replacing it so rapid native scroll events cannot orphan an older callback
• reuse the same cleanup when a new programmatic scroll supersedes pending completion work
• clear both stored handles after cancellation, including valid handle 0

Root cause

checkFinishedScrollFallback stores its watchdog in timeoutCheckFinishedScrollFallback, outside the generic state.timeouts registry that unmount cleanup clears. The completion animation frame is stored separately as well. Both handles could therefore survive teardown and call back into an unmounted list.

checkFinishedScroll could also overwrite the stored animation-frame ID when multiple native scroll events arrived before the next frame. Coalescing completion checks to the latest frame prevents an older callback from becoming unreachable by cleanup.

This addresses the reproduced post-unmount watchdog failure in #508. #485 is complementary work for fallback re-arming during a live session.

Validation

• pnpm dlx bun test __tests__/core/checkFinishedScroll.test.ts __tests__/core/scrollTo.test.ts __tests__/components/LegendList.props.test.tsx (67 passed)
• pnpm dlx bun run tsc:src
• pnpm dlx bun run lint:fix
• pnpm dlx bun run lint
• pnpm dlx bun test (1,514 passed)
• pnpm dlx bun run build

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