v0.172.0 — R-75: canonical import root, catalog-derived skeleton, import surfaces

${IMPORT_PATH} = <system namespace root>/userdata/import — ONE drop-zone per box,
on the system drive, injected at BOTH compose-env builders with NO per-drive
fallback (unresolvable leaves it unset so compose fails loudly rather than
quietly building a second, dead drop-zone).

Third BindRoot (RootImport) + Import list in BackupSpec, extended through
ValidateBackupSpec/ClassifyBinds. Load-bearing: a stale `userdata: import/<app>`
entry against the moved bind would be a WHOLE-BLOCK reject, taking the app's
mandatory hdd classification with it.

Exhaustive-root audit: resolveAbs/structuralGuard/ComputeCaptureSet/
ComputeFabBuckets now take importRoot explicitly (an import bind resolved
against hddPath would name a directory on the wrong drive); unresolvable is
refused loudly into Skipped. GetImportRoot added to both provider interfaces.

Catalog-derived skeleton: UserdataSkeleton() -> UserdataSkeletonCarry() +
BuildUserdataSkeleton(), SORTED. The carry-list makes zero-removals true by
construction (`documents` is in no catalog app but on both boxes) and is the
fresh-box floor. The sort is not tidiness: the naive map-order derivation
measured 20 distinct outputs from 20 identical runs, which with fbNeedsRecreate
is a fleet-wide FileBrowser restart loop.

One authoritative compose parser: ParseComposeUserdataMounts now delegates to
ParseComposeClassifiableBinds. Import root excluded from per-app migration.

Surfaces: FileBrowser /srv/beolvasas source; app-page "Hova tegyem a fajlokat?"
with PathEscape deep links (never QueryEscape) and class-driven copy;
data_paths: annotation with the Fork-3 asymmetry; system-owned beolvasas SMB
share refused server-side at handler AND store, button omitted in template.

Caught on the way: the sharing template's row struct was function-local, so
adding {{if .System}} would have 500'd every share row. ShareRow is now
package-level and the render test uses the handler's own type.

Tests 915 -> 949, all green. MinAgent unchanged.

controller/cmd/controller/main.go

@@ -1328,6 +1328,11 @@ func (a *stackAdapter) GetDockerVolumes(name string) []string {
 	return backup.ResolveDockerVolumeNames(s.ComposePath)
 }
 
+// GetImportRoot delegates to the stack manager's canonical drop-zone root (R-75). Both the backup
+// and the export provider need it: ${IMPORT_PATH} binds live on the SYSTEM drive and can NOT be
+// resolved from an app's HDD_PATH.
+func (a *stackAdapter) GetImportRoot() string { return a.mgr.GetImportRoot() }
+
 func (a *stackAdapter) GetStackHDDPath(name string) string {
 	s, ok := a.mgr.GetStack(name)
 	if !ok {
@@ -1543,6 +1548,11 @@ func (a *exportAdapter) GetStackHDDMounts(name string) []string {
 	return nil
 }
 
+// GetImportRoot delegates to the stack manager's canonical drop-zone root (R-75). Both the backup
+// and the export provider need it: ${IMPORT_PATH} binds live on the SYSTEM drive and can NOT be
+// resolved from an app's HDD_PATH.
+func (a *exportAdapter) GetImportRoot() string { return a.mgr.GetImportRoot() }
+
 func (a *exportAdapter) GetStackHDDPath(name string) string {
 	s, ok := a.mgr.GetStack(name)
 	if !ok {