R-203: the app and its backup look in the same directory — one resolver, every caller

appbackup's path helpers take a NAMESPACE ROOT. Five call sites passed a bare DRIVE path.
On an enrolled drive the two coincide, so nothing showed; on the system-data fallback they
differ by exactly the felhom-data segment, and the app then bound a directory the off-site
capture set never looked at -- while the run reported ok. Measured live on demo-hp: the app
wrote to /mnt/sys_drive/userdata/media/books, the capture set looked for
/mnt/sys_drive/felhom-data/userdata/media/books.

THE RULE NOW HAS ONE EXPRESSION. appbackup.NamespaceRootFor / IsEnrolledDrive encode the
drive-kind comparison; backup.Manager.namespaceRoot and stacks.Manager.inGuest delegate to
it. There were already TWO copies and they differed -- the backup package's compared without
filepath.Clean, the stacks package's with it, so a trailing slash from config would have
flipped the mode in one and not the other.

Sites routed through it:
  - stacks/deploy.go withPathVars -> ${USERDATA_PATH}   (the live defect)
  - appexport/fabplan.go + export.go                     (via a new provider method)
  - web/handlers.go FileBrowser mounts                   (latent: the system drive is
    deliberately never a registered StoragePath, so this is the identity today)

ComputeFabBuckets now receives the namespace root, which is what ComputeCaptureSet has always
received -- so the export's classified paths and the backup's capture set describe the same
directories by construction instead of by coincidence.

Tests are table-driven over BOTH drive kinds, because this survived by being invisible on the
kind that already worked. Red-proofs observed: restoring the bare-path call fails the
system-drive row with the two paths differing by /felhom-data; inverting the drive-kind
comparison fails every enrolled row.

controller/internal/appexport/export.go

@@ -659,7 +659,9 @@ func (e *Exporter) exportHDDData(req ExportRequest, dataDir string, manifest *Ma
 	// Task 4: the class-scoped plan. Legacy / no-block apps get an EMPTY plan (all mounts kept, root
 	// tar with zero excludes) → byte-identical v0.130.0 capture.
 	plan := e.computeFabPlan(req, mounts)
-	ud := appbackup.UserdataDir(filepath.Clean(e.provider.GetStackHDDPath(stackName)))
+	// R-203: a NAMESPACE ROOT, not the drive path (identical on an enrolled drive; one segment short
+	// on the system-data fallback).
+	ud := appbackup.UserdataDir(filepath.Clean(e.provider.GetStackNamespaceRoot(stackName)))
 
 	claimed := make(map[string]string) // subdir → mount that claimed it
 	for _, mount := range mounts {