fix: preserve newer pending file reads - #844

Summary

• Make pending-read cleanup owner-aware on the mtime invalidation path.
• Only remove a pending read when the map still points to the same Promise.
• Add a deterministic regression test that prevents an older read from clearing a replacement read.

Fixes #840

Validation

• pnpm run test:ava source/utils/file-cache.spec.ts — 19 passed
• pnpm test:format — passed
• pnpm test:types — passed
• pnpm test:lint — passed
• pnpm test:knip — passed
• pnpm audit --registry=https://registry.npmjs.org --audit-level=high --ignore-unfixable — passed

The repository's pnpm run test:all wrapper cannot start on Windows because it invokes ./scripts/test.sh. Running the equivalent commands individually reached existing full-suite failures/timeouts in acp-content.spec.ts and ai-sdk-client tests; the focused file-cache suite passes.