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/observe.go

@@ -161,14 +161,19 @@ func (o *Observer) snapshot(ctx context.Context) ([]observed, error) {
 func (o *Observer) build(s proxmox.Storage, mounts []Mount) observed {
 	category := categorize(s.Type)
 
-	// Resolve the backing device + mount path for dir-like targets.
+	// Resolve the backing device + mount path for dir-like targets — ONLY from the
+	// target's OWN mountpoint. We deliberately do NOT fall through to the containing
+	// filesystem (e.g. root): an unmounted removable dir-storage's mountpoint reverts to a
+	// bare directory on root, and resolving its UUID/durable_id to ROOT's UUID would be a
+	// catastrophic DR mis-id (the hub would re-attach the wrong disk). When the target is
+	// not its own mount, we leave the device/UUID unknown and the durable_id falls back to a
+	// stable store id (never another fs's UUID). The authoritative UUID for re-attach comes
+	// from a prior attached observation (watchdog memory) or the hub manifest (slice 10).
 	var backingDevice, mountPath string
 	var exactMount bool
 	if category == catDir {
 		if dev, mp, ok := exactMountDevice(mounts, s.Path); ok {
 			backingDevice, mountPath, exactMount = dev, mp, true
-		} else if dev, ok := containingMountDevice(mounts, s.Path); ok {
-			backingDevice = dev // for the class hint only; not its own mount
 		}
 	}
 
@@ -396,25 +401,6 @@ func exactMountDevice(mounts []Mount, path string) (device, mountPoint string, o
 	return "", "", false
 }
 
-// containingMountDevice finds the device of the longest mountpoint that is a prefix of
-// path (the filesystem that path lives on) — used only for the class-hint disk lookup.
-func containingMountDevice(mounts []Mount, path string) (device string, ok bool) {
-	if path == "" {
-		return "", false
-	}
-	clean := cleanMountPath(path)
-	best := -1
-	for _, m := range mounts {
-		mp := cleanMountPath(m.MountPoint)
-		if clean == mp || strings.HasPrefix(clean, mp+"/") || mp == "/" {
-			if len(mp) > best {
-				best, device, ok = len(mp), m.Device, true
-			}
-		}
-	}
-	return device, ok
-}
-
 func cleanMountPath(p string) string {
 	p = strings.TrimRight(p, "/")
 	if p == "" {