test: add NonceManager with per-account lock and concurrency tests (#6)

Implements the nonce module needed for issue #5 acceptance criteria:
- src/nonce/lock.ts: FIFO async mutex keyed by account (fixes vero-core-engine#164, #183)
- src/nonce/index.ts: NonceManager with atomic reserve() and refresh()
- src/__tests__/nonce.concurrency.test.ts: concurrency regression tests

Concurrency tests cover:
- N concurrent reserve() calls yield N distinct, gapless sequences
- Concurrent first-reservations do not duplicate sequences
- Interleaved refresh() + reserve() produce no duplicates (vero-core-engine#183)
- Different accounts proceed in parallel, not serially
- High-concurrency (50 callers) scenario

All checks pass: npm test (62/62), typecheck, lint, build.

Closes #5

Author: DatabyPrisca