agent v0.35.0: intermediary mount — guest-reboot re-propagation (load-bearing)

A guest's parent bind is non-recursive, so a guest reboot leaves enrolled drives
bound on the HOST but invisible in the fresh guest ns (propagation only delivers
new events). AttachDrive(vmid) now checks GuestSeesMount (/proc/<pid>/mountinfo)
and force re-binds (umount+mount) to re-propagate; a 20s periodic reconcile
self-heals guest reboots without an agent restart; BoundUnderParent reflects guest
visibility (the controller gate's signal). Caught + fixed in the live migration.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/localapi/disks_test.go

@@ -412,7 +412,7 @@ func (f *fakeGuestAttacher) AttachBind(_ context.Context, vmid int, mountKey, wh
 }
 func (f *fakeGuestAttacher) count() int { f.mu.Lock(); defer f.mu.Unlock(); return len(f.calls) }
 
-func (f *fakeGuestAttacher) AttachDrive(_ context.Context, where string) (string, error) {
+func (f *fakeGuestAttacher) AttachDrive(_ context.Context, _ int, where string) (string, error) {
 	f.mu.Lock()
 	defer f.mu.Unlock()
 	if f.attachDriveFail {
@@ -421,6 +421,7 @@ func (f *fakeGuestAttacher) AttachDrive(_ context.Context, where string) (string
 	f.attachDrives = append(f.attachDrives, where)
 	return StablePathForRaw(where), nil
 }
+func (f *fakeGuestAttacher) GuestSeesMount(_ context.Context, _ int, _ string) bool { return true }
 func (f *fakeGuestAttacher) attachDriveCount() int {
 	f.mu.Lock()
 	defer f.mu.Unlock()