Move host.unload() before Stage 1 in stop() so the shutdown monitor
task is cancelled before infrastructure teardown begins. This prevents
a race where the monitor could react to PrepareForShutdown after
stop() has already started tearing down.
For the remaining edge case where shutdown() is called while stop()
is running, log a warning and return immediately instead of awaiting
the shutdown event (which would deadlock since stop() never sets it).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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