* fix(channels): retry the runtime config file lock instead of failing after ~10s on Windows
The runtime channel config sidecar lock (#6844) used Windows' blocking msvcrt LK_LOCK mode, which gives up after roughly ten seconds and surfaces cross-worker contention as Permission denied instead of serializing the read-modify-write cycles it exists to serialize, while fcntl.flock(LOCK_EX) waits on POSIX. Retry the non-blocking acquisition (LK_NBLCK + 0.2s) on the Windows fallback, mirroring the extensions manager's lock (#6020), and bind fcntl/msvcrt at module level so the fallback branch is testable.
Tests: new backend/tests/test_channel_runtime_config_store_lock.py (5 tests: retry cadence via fake msvcrt, sidecar byte creation, POSIX flock preservation, direct Windows regression through the sys.modules seam, real cross-thread contention). 3 of them fail on the unmodified base (one with OSError 36 raised by the blocking call itself) and all pass with the fix. Related suites: store peers + connections router 86 passed / 1 skipped; blocking_io + encoding + channels 498 passed / 2 skipped. Ruff format and check clean on both files. Windows 11, Python 3.12 (uv-managed venv).
* fix(channels): share contention retries and diagnose prolonged lock waits
* docs(channels): shorten guidance to satisfy lint budget
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
