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>

docs/ARCHITECTURE.md

@@ -105,7 +105,8 @@ then normal rotation.
   changes. The controller refreshes its cached settings; enabling or
   disabling a plugin queues `_reconcile_enabled_plugins()`, which loads or
   unloads it on the display thread; each plugin gets `on_config_change()`
-  for its own section. Set `LEDMATRIX_HOT_RELOAD=false` to turn this off.
+  for its own section, under its plugin lock
+  (`PluginManager.apply_config_change()`). Set `LEDMATRIX_HOT_RELOAD=false` to turn this off.
   Matrix hardware settings are only read at start-up.
 - **Vegas mode.** [`src/vegas_mode/`](../src/vegas_mode/): the display loop
   calls `VegasModeCoordinator.run_iteration()`