UpdateMarkerGuard::acquire overwrote the in-progress marker
unconditionally, so a Tauri update launched while a dashboard-spawned
"hermes update" was mid-flight simply took the marker and ran a second
updater over the same checkout. That is the race behind the reported
Windows failure: install-mode bootstrap rewound the tree while the
dashboard's updater was still running npm install against it.
acquire now returns Result and refuses when a live foreign owner holds
the marker, and Drop no longer deletes a marker this process does not
own. Liveness matches the Python and Electron readers of the same file:
dead pid or past the shared age ceiling means stale and reclaimable, so
a crashed updater cannot wedge future updates.
Adds a cfg(unix) libc dependency for the signal-0 liveness probe; the
Windows path uses OpenProcess/GetExitCodeProcess.
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