Add read consistency and replica lag routing

[jdestep93]

Summary

• add explicit read-after-write consistency helpers with async-context writer pinning
• add user-provided replica lag probes and reader state reporting
• exclude stale readers from read scheduling while preserving existing fallback behavior
• document the new opt-in APIs and ignore local .codex state

Closes #1

Validation

• npm run typecheck
• npm run test:unit
• npm run build
• npm run test:integration