v0.5.1: live-validation prep — fix unmounted-dir durable_id mis-id + watchdog UUID memory

Surfaced preparing the live USB validation on demo-felhom:
- observe.go: an unmounted removable dir-storage no longer falls through to the ROOT fs
  for its backing device/UUID — durable_id was becoming uuid:<root-uuid> (a DR mis-id that
  would re-attach the wrong disk). Now derived only from the target's own mountpoint;
  unmounted → no device + stable store:<name> durable_id. Removed containingMountDevice.
- watchdog.go: remember the fs-UUID observed while attached and backfill it onto the
  re-mount target, so re-mount works even if the known-set cache refreshed mid-drop
  (doc 03 §7 "sourced from the existing definition").

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

internal/storage/watchdog_test.go

@@ -147,12 +147,14 @@ func TestWatchdog_DebounceCoalescesFlaps(t *testing.T) {
 type fakeRemounter struct {
 	mu    sync.Mutex
 	calls []string
+	uuids []string
 }
 
 func (r *fakeRemounter) Remount(_ context.Context, t KnownTarget) {
 	r.mu.Lock()
 	defer r.mu.Unlock()
 	r.calls = append(r.calls, t.Name)
+	r.uuids = append(r.uuids, t.UUID)
 }
 func (r *fakeRemounter) count() int {
 	r.mu.Lock()
@@ -208,6 +210,34 @@ func TestWatchdog_ReMountOnDeviceReturn(t *testing.T) {
 	}
 }
 
+func TestWatchdog_ReMountUsesRememberedUUID(t *testing.T) {
+	// The re-mount key must survive the known-set cache losing the UUID mid-drop (an
+	// unmounted dir-storage can't resolve its own UUID). The watchdog remembers the UUID
+	// observed while attached and backfills it onto the re-mount target.
+	known := &staticKnown{targets: []KnownTarget{{Name: "usb", MountBacked: true, UUID: "277a2179", MountPath: "/mnt/usb"}}}
+	live := &mapLiveness{present: map[string]bool{"usb": true}, device: map[string]bool{"usb": true}}
+	rem := &fakeRemounter{}
+	w, _, _ := newTestWatchdog(known, live, 30*time.Second)
+	w.remounter = rem
+	w.spawn = func(f func()) { f() }
+	ctx := context.Background()
+
+	w.tick(ctx) // baseline attached → remembers UUID 277a2179
+
+	// Cache refreshes during the drop and loses the UUID (the unmounted observe can't
+	// resolve it); device still present, unmounted.
+	known.targets = []KnownTarget{{Name: "usb", MountBacked: true, UUID: "", MountPath: "/mnt/usb"}}
+	live.set("usb", false)
+	w.tick(ctx)
+
+	if rem.count() != 1 {
+		t.Fatalf("expected one re-mount, got %d", rem.count())
+	}
+	if rem.uuids[0] != "277a2179" {
+		t.Fatalf("re-mount must use the remembered UUID, got %q", rem.uuids[0])
+	}
+}
+
 func TestWatchdog_ReadErrorSkipsTick(t *testing.T) {
 	known := &staticKnown{err: errors.New("proxmox blip")}
 	live := &mapLiveness{present: map[string]bool{}}