Destroy consumers of rejected duplicates, keep state updaters pure

[rmuratov]

Closes #23
Closes #24

What

Reworks useSourcemapsStore so that SourceMapConsumer lifecycle management happens outside the React state updaters.

• Memory leak: SourceMapConsumer never destroyed for rejected duplicates #23: Candidates rejected by deduplication hold a live WASM-backed consumer; it is now destroy()ed instead of leaking on every re-upload of the same file. Duplicates within one file selection are also deduplicated now (previously two identical files picked together were both added).
• Side effect inside setState updater in deleteSourceMap #24: deleteSourceMap called target.consumer.destroy() inside the setSourceMaps updater. Updaters must be pure — StrictMode invokes them twice, calling destroy() twice. The lookup and destroy() now happen in the event handler; the updater only filters.

Deliberately not destroying consumers on unmount: StrictMode's dev-only remount preserves state while running effect cleanups, so an unmount cleanup would destroy consumers that are still referenced after the remount. The store lives in the root component, where unmount means page unload anyway.

Tests

Three new integration tests in the source map lifecycle suite spy on destroy() via the prototype of the concrete consumer class:

• re-uploading the same file destroys the rejected duplicate's consumer and keeps one list item,
• selecting two identical files together adds the map once and destroys the extra consumer,
• deleting a source map under StrictMode destroys the consumer exactly once.

All three fail against the previous implementation and pass with the fix; the full suite is 29/29.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (5be63c6) to head (3284631).

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #36   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines          203       208    +5     
  Branches        51        52    +1     
=========================================
+ Hits           203       208    +5     

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[rmuratov]

Added a follow-up commit that restores 100% coverage and enforces it via coverage.thresholds: { 100: true } in vite.config.ts (CI runs npm run coverage, so any future drop below 100% fails the build).

The three uncovered spots and how they were handled:

• use-sourcemaps-store.ts — the if (!target) return guard in deleteSourceMap was unreachable through the UI (delete buttons only exist for rendered items; this gap existed on main too). Replaced the not-found special case with a loop that destroys matching consumers — both branch outcomes are exercised by existing tests.
• lib.ts — the position.column == null ? null : … arm in toUnifiedPosition was dead code: when originalPositionFor finds a mapping, source, line, and column are always present. Replaced the ternary with an isResolvedPosition type guard in tryGetOriginalPosition, so toUnifiedPosition now takes a fully resolved position and the dead branch is gone.
• app.tsx — the non-Enter path of the file-upload button's keydown handler (uncovered since before this PR) is now covered by firing a KeyA keydown and asserting the file selector does not open.