plugin: parallel release fetches, faster shutdown, uninstall runner stop

- Run Plugin Manager release fetches on OS threads so they execute in

  parallel instead of serialising on the async executor.

- Reduce V4Connection shutdown poll from 3s to 100ms; the runner is killed

  and reaped without blocking host shutdown.

- Add PluginManager::remove and PluginProcess::shutdown_and_wait so

  uninstall can stop the runner before deleting files, fixing Windows

  access-denied errors on plugin removal.

- Rebuild ribbon immediately after uninstall so the tab disappears.

crates/ocs_plugin_api/src/process/v4.rs

@@ -302,10 +302,12 @@ impl V4Connection {
         }
         // Dropping the writer closes the socket. The reader thread will then
         // exit and clear `alive`. Joining can deadlock if a plugin child
-        // inherited the socket handle, so we only poll briefly.
-        let deadline = Instant::now() + Duration::from_secs(3);
+        // inherited the socket handle, so we only poll briefly. The host process
+        // will kill the runner afterwards, so a long wait only delays shutdown
+        // without improving cleanup.
+        let deadline = Instant::now() + Duration::from_millis(100);
         while self.shared.alive.load(Ordering::SeqCst) && Instant::now() < deadline {
-            std::thread::sleep(Duration::from_millis(50));
+            std::thread::sleep(Duration::from_millis(10));
         }
         let _ = self.reader_handle.lock().unwrap_or_else(|e| e.into_inner()).take();
     }