vmm: guest event persistence holds the global state lock and blocks the async executor #1269

Problem

HostApi.Notify calls synchronous App::vm_event_report on an async worker. An instance.info event executes safe_write (including file and directory fsync) while holding the global AppState mutex. A shutdown.progress event reporting powering off also writes its started marker under that mutex.

Slow storage therefore delays unrelated VMs’ control-plane state operations as well as the async worker. This is a source-level finding, not a measured production latency claim.

Fix

Move persistence off the executor and outside the global state lock. Preserve per-VM write ordering and serialize with VM removal, including when the notification request is cancelled. Add regression tests for responsiveness and lifecycle safety.

The supervisor log fsync changes in #1263 do not cover this call path.