tear down the previous workspace's services on project switch (T-367)

buildDispatcher composed a fresh PaneRegistry, FilesService,
SearchService, and EditorRegistry per workspace, but their shutdown()
methods had zero callers — every project switch left the old set's
file watcher emitting into the new workspace's bus and its PTYs
alive. The dispatcher now pairs with a teardown closure that the
serialized swap invokes after the old server stops; the same-path
reuse fast-path drops the unused new set without teardown since its
services are inert until a command starts them. SearchService gains
the shutdown() it was missing (cancels in-flight searches).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/src/daemon/search_commands.dart

@@ -52,6 +52,15 @@ class SearchService {
     _active.remove(id)?.cancel();
   }
 
+  /// Cancel every in-flight search. Called when the workspace service
+  /// set is torn down on project switch (T-367).
+  Future<void> shutdown() async {
+    for (final c in _active.values) {
+      c.cancel();
+    }
+    _active.clear();
+  }
+
   /// Compute (preview) or perform (apply) a search-and-replace.
   ///
   /// Preview returns per-file before/after edits without touching disk.