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.go

@@ -86,6 +86,7 @@ type Watchdog struct {
 	fired       bool                 // lastFire is valid
 	pending     bool                 // a transition is awaiting the debounce window
 	lastRemount map[string]time.Time // name -> last re-mount dispatch (rate-limit)
+	lastUUID    map[string]string    // name -> last fs-UUID observed while ATTACHED (re-mount key)
 }
 
 // WatchdogOptions configures a Watchdog. Targets, Liveness and Trigger are required; the
@@ -131,6 +132,7 @@ func NewWatchdog(opts WatchdogOptions) *Watchdog {
 		spawn:       func(f func()) { go f() },
 		last:        map[string]bool{},
 		lastRemount: map[string]time.Time{},
+		lastUUID:    map[string]string{},
 	}
 }
 
@@ -169,6 +171,21 @@ func (w *Watchdog) tick(ctx context.Context) {
 		return
 	}
 
+	// Remember each target's fs-UUID while it is observable (attached), and backfill it
+	// onto a target the current observe couldn't resolve (the unmounted case loses it). The
+	// re-mount key is "sourced from the existing definition" (doc 03 §7): the agent learns
+	// the UUID while attached, so a drop+return cycle can re-mount by-UUID even after the
+	// known-set cache refreshed mid-drop. Single-goroutine (tick), guarded for -race.
+	w.mu.Lock()
+	for i := range known {
+		if known[i].UUID != "" {
+			w.lastUUID[known[i].Name] = known[i].UUID
+		} else if u := w.lastUUID[known[i].Name]; u != "" {
+			known[i].UUID = u
+		}
+	}
+	w.mu.Unlock()
+
 	// Probe outside the lock.
 	type probe struct {
 		t             KnownTarget