fix(plugins): one hung plugin no longer stops every plugin from updating (#677)

The update worker no longer blocks forever on a plugin whose display()
never returns. It waits at most PLUGIN_LOCK_TIMEOUT (5s) for a plugin's
lock, then skips that plugin's update (a report-only "busy skip" in
health) and keeps updating every other plugin. display() frames are timed
(slow calls logged and counted; calls past the executor timeout recorded as
hangs), a hung update() is recorded, and on_config_change() now runs under
the plugin lock or is deferred to the worker. The plugin-facing API is
unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/test_on_demand_disabled_plugin.py

@@ -260,8 +260,11 @@ class TestReleasingThePlugin:
 
         callback({}, {'enabled': False, 'color': 'red'})
 
+        # The change goes through the manager's locked apply_config_change
+        # (which calls on_config_change under the plugin's lock).
         plugin = controller.plugin_modes['preview_a']
-        plugin.on_config_change.assert_called_once_with({'enabled': True, 'color': 'red'})
+        controller.plugin_manager.apply_config_change.assert_called_once_with(
+            'preview-me', {'enabled': True, 'color': 'red'}, plugin_instance=plugin)
 
 
 class TestRestoredSession: