refactor(plugins): the display publishes plugin runtime state; retire plugin_state.json (#690)

Stage 2 of the web plugin catalog, after #688.

- The display publishes a plugin runtime snapshot (plugin_runtime.py) to
  the shared cache: per plugin loaded, lifecycle state, a short redacted
  error summary, the version it loaded and when, plus published_at /
  stale_after / running. Written on change (throttled to 10 s; the
  RUNNING/ENABLED flip of an ordinary update is not a change) and once a
  minute otherwise; cleanup() publishes running: false.
- The web reads it back and restores loaded / state / error_info in
  /api/v3/plugins/installed (plus loaded_version, loaded_at and
  data.runtime). Only a live snapshot counts; stale, stopped or missing
  answers null and says which.
- data/plugin_state.json is retired: every reader and writer moved to
  config + disk (desired) or the snapshot (observed). Nothing in it was
  non-derivable, so nothing is migrated and an existing file is left
  unread. The web-side PluginStateManager (state_manager.py) is removed;
  the display's plugin_state.PluginStateManager is the only state machine.
- StateReconciliation compares config + disk with the snapshot, reporting
  enabled-but-not-loaded and older-version-loaded as no_action findings.
- Backups list installed manifests with enabled from config.json.

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

src/plugin_system/plugin_manager.py

@@ -522,7 +522,13 @@ class PluginManager:
                         raise
             else:
                 self.state_manager.set_state(plugin_id, PluginState.DISABLED)
-            
+
+            # The version this instance runs, for the runtime snapshot the
+            # web UI reads: the manifest on disk can move on after an update.
+            version = manifest.get('version')
+            self.state_manager.record_loaded(
+                plugin_id, version if isinstance(version, str) else None)
+
             self.logger.info("Loaded plugin: %s", plugin_id)
             
             return True
@@ -774,6 +780,9 @@ class PluginManager:
         except Exception as e:
             self.logger.error("Error unloading plugin %s: %s", plugin_id, e, exc_info=True)
             self.state_manager.set_state(plugin_id, PluginState.ERROR, error=e)
+            if plugin_id not in self.plugins:
+                # Failed after the instance was dropped: it is not loaded.
+                self.state_manager.record_unloaded(plugin_id)
             return False
     
     def reload_plugin(self, plugin_id: str) -> bool: