refactor: clear Persist Engine quality gate

[jonbaldie]

Closes #34.

What changed

• Replaced assignment inside the file-read loop with an explicit read/break cycle.
• Renamed the caught error and removed MemoryStore’s unused no-op dir parameter.
• Added ordered multi-event MemoryStore replay coverage.
• Added real cross-process FileStore lock contention coverage: a child saveEvent must block until the parent releases the OS file lock.
• Added no suppression, exclusion, policy change, or mock.

TDD evidence

• Persist gate red on IfStatementAssignment, _e, and _dir; green after the minimal refactor.
• Reversing MemoryStore replay makes the new order test fail.
• Removing FileStore.saveEvent locking makes the child complete while the parent lock is held and the new lock test fail.
• Restored production passes both canaries.

Validation

• Full Deno suite: 180 passed, including real socket E2E and child-process lock contention.
• Persist/manager focused suite: 49 passed.
• quality:unit -- persist-engine exits 0.
• deno lint src/persist.ts passes.
• Two-axis standards/spec review passes with no findings.
• GitHub Actions will run Test → Mutasaurus → Stryker.