Remind periodically while a context lock acquisition is blocked

[fzyzcjy]

Part of #1837

[gemini-code-assist]

Caution

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[guapisolo]

[P2][unresolved at safe checkpoint #2176 and checked tip #2649] Please avoid spawning a reminder task on every uncontended acquire. Because asyncio.Lock.acquire() can complete without yielding, a tight acquire/release sequence leaves one cancel-requested task and ready callback per call until the loop runs again; on this head, 100,000 calls retained about 91.8 MB before the next yield. Please preserve an allocation-free uncontended path and reliably drain the contended-path reminder without weakening lock fairness or cancellation safety.

[guapisolo]

[P2][unresolved at safe checkpoint #2176 and checked tip #2649] Please wait on observed reminder events instead of assuming two 10 ms timers run within this 80 ms window. Under a busy or stalled CI event loop, the test sleep and first reminder can become runnable together, so the waiter is cancelled after only one log and this test flakes despite correct reminder behavior; a deterministic 100 ms event-loop stall reproduces it on this head.