Serialize memvid access with cross‑process lock

[rafaelcalleja]

Fixes #3

Summary

• Add cross‑process file lock around Mind.open() and all memvid operations to prevent concurrent access to .mv2.
• Keep corruption handling, but now it runs under lock, so Invalid indicates real corruption.
• Add regression test that fails on main and passes with the lock.

Changes

• New src/utils/memvid-lock.ts using proper-lockfile
• Lock wrapping in src/core/mind.ts
• New test src/__tests__/mind-lock.test.ts
• Add deps proper-lockfile and @types/proper-lockfile

Testing

• npm test -- src/__tests__/mind-lock.test.ts
• npm run typecheck

TODO

• Re‑run the reproduction steps from issue Race condition in concurrent hooks corrupts mind.mv2 (Invalid sketch track magic) #3 to confirm they no longer fail (no “Invalid sketch track magic”).