Gate concurrent warm synchronization

[chubes4]

Summary

• give primary warm synchronization one non-blocking process owner per runtime index
• let concurrent boots immediately retain the last complete index instead of entering SQLite writer contention
• keep ownership process-safe through OS flock, which releases automatically when the synchronizer exits
• expose synchronizer_active evidence when a boot deliberately skips synchronization

Closes #248.

Verification

• all PHP files pass php -l
• all tests/smoke-*.php scripts pass
• multi-process regression proves one synchronization owner, immediate contender fallback, clean release, and later reacquisition
• focused warm-sync atomic, hydration concurrency, and primary storage runtime tests pass

AI assistance

OpenAI GPT-5.6 Sol via OpenCode reviewed MDI primary boot and persistence boundaries, implemented the synchronization ownership gate and multi-process regression coverage, and ran repository verification. Chris Huber directed the follow-up architecture and remains responsible for the change.