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.

src/plugin/external.rs

@@ -267,7 +267,7 @@ fn parse_string_array(s: &str) -> Vec<String> {
 pub(crate) use loader::{shutdown_plugins, with_manager};
 
 #[cfg(all(not(target_arch = "wasm32"), not(test)))]
-pub(crate) use loader::{load_at_startup, loaded_ids};
+pub(crate) use loader::{load_at_startup, loaded_ids, remove_plugin};
 
 #[cfg(not(target_arch = "wasm32"))]
 #[cfg_attr(test, allow(dead_code))]
@@ -342,6 +342,19 @@ mod loader {
         });
     }
 
+    /// Shut down the loaded plugin with `id` and remove it from the manager so
+    /// its files can be deleted on Windows. Returns true if the plugin was
+    /// loaded and has been removed.
+    pub fn remove_plugin(id: &str) -> bool {
+        MANAGER.with(|m| {
+            if let Some(manager) = m.borrow_mut().as_mut() {
+                manager.remove(id)
+            } else {
+                false
+            }
+        })
+    }
+
     /// Path to the native library beside `plugin.toml`, if any.
     fn lib_file(dir: &Path) -> Option<PathBuf> {
         let ext = lib_extension();