fix: ignore stale preview renders

[PathGao]

Fixes #253.

Summary

• Give each split/edit preview request a revision.
• Cancel its debounce timer when the reactive effect is invalidated.
• Apply a completed render only when the same active tab and raw content still match the request.

Validation

• node --test --import tsx scripts/previewRenderRevision.test.ts
• npm run check

[PathGao]

@alecdotdev This branch has been rebased onto the current master and force-pushed; the merge conflict is resolved. Validation passed: node --test --import tsx scripts/previewRenderRevision.test.ts and npm run check. This is the first PR in the dependency chain; please merge it before #258, #260, and #266.