feat(users): per-user sealed folders users/<id>/{home,apps,data,permissions}; storage SET_USER/LOCK/PURGE; init sign-in/sign-out loop (purge fresh/removed/guest, seed from factory, session start/stop); shell logout; pm user+system app shelves; terminal SIGNED_OUT; per-program stack (gatekeeper 12p); 128K load scratch

crates/servers/storage-server/src/adapters/mock_fs.rs

@@ -139,4 +139,14 @@ impl FileSystem for MockFileSystem {
     fn rmdir(&mut self, _path: &[u8]) -> bool {
         false
     }
+
+    /// Flat: a tree is the files named under it.
+    fn purge(&mut self, path: &[u8]) -> bool {
+        for slot in self.files.iter_mut() {
+            if slot.as_ref().is_some_and(|f| f.name.as_bytes().starts_with(path)) {
+                *slot = None;
+            }
+        }
+        true
+    }
 }