fix: contain log sidecar persistence failures

[codeforester]

Fixes #410. Keeps ordinary sidecar open, lock, unlock, and close failures inside a non-recursive diagnostic boundary while preserving process-control exceptions. Tests: tests/test_logging_hot_path.py.

[codeforester]

Review: not ready; 4 CI jobs fail (Click 8.1 / Python 3.10 and 3.14) on test_logging_sidecar_io_failure_does_not_fail_attached_json_command. Under Click 8.1 result.output holds stdout only, so assert "logging persistence failed" in result.output fails (output was just the JSON envelope). Assert on result.stderr as the other new tests do.

The production change looks right (non-recursive diagnostic, process-control exceptions preserved, close() hardened). Non-blocking: every failing record now writes a stderr line, so a persistently broken sidecar will print one line per log call; consider emitting once per handler. The tests also reach into private handler state (_lock_stream, _lock_identity), which is brittle; a small test helper would help.