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_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"io"
 	"log/slog"
+	"strings"
 	"testing"
 
 	"gitea.dooplex.hu/admin/felhom-agent/internal/hub"
@@ -249,6 +250,15 @@ func TestObserve_USBUnpluggedIsDisconnected(t *testing.T) {
 	if usb.DurableID == "" {
 		t.Errorf("durable_id must never be empty (DR re-attach lookup)")
 	}
+	// CRITICAL: an unmounted dir-storage must NOT inherit the ROOT filesystem's UUID/device
+	// (that would re-attach the WRONG disk on DR). With only root mounted, the USB target
+	// resolves to no device and a stable store-id durable_id — never "uuid:<root-uuid>".
+	if strings.HasPrefix(usb.DurableID, "uuid:") {
+		t.Errorf("unmounted target must not carry a uuid durable_id (got %q — root-UUID mis-id risk)", usb.DurableID)
+	}
+	if usb.BackingDevice != "" {
+		t.Errorf("unmounted target must not resolve a backing device (got %q)", usb.BackingDevice)
+	}
 }
 
 func TestObserve_ProxmoxErrorIsFatalForStorage(t *testing.T) {