fix: refresh local db state before writes

[omribz156]

Summary

• refresh LocalDb from disk immediately before set and delete
• add regression coverage for two LocalDb instances writing/deleting against the same backing file

This addresses #71. Previously, a LocalDb instance could keep a stale in-memory snapshot from construction time and then overwrite keys written by another instance when it later saved its own snapshot.

Verification

• yarn install --frozen-lockfile --ignore-scripts
• yarn test tests/utils/local-db.test.ts --runInBand
• npx prettier --check lib/utils/local-db.ts tests/utils/local-db.test.ts
• npx eslint lib/utils/local-db.ts tests/utils/local-db.test.ts --ext .ts --config .eslintrc.json
• yarn build:esm
• yarn build:cjs
• git diff --check

Notes: ESLint reports the existing no-explicit-any warnings in lib/utils/local-db.ts; there are no lint errors from this change. git diff --check only reported local CRLF warnings on Windows.

This was implemented with Codex assistance, with the final diff manually reviewed and kept focused.