fix(plugin-system): load/enable failures, atomic state files, pip lock, test-double parity (#645)

- load_plugin: an on_enable() that raises unregisters the instance, so the
  next load retries instead of returning True "already loaded".
- get_plugin_info: guard plugin.get_info(); one plugin raising no longer
  breaks /api/v3/plugins/installed.
- plugin_state.json and the operation history are written with
  atomic_write_text under their lock.
- plugin_loader: module-level lock serialises pip installs across the
  parallel startup loaders.
- store_manager._install_via_download: extract dir cleanup moved to finally.
- Test doubles: draw_image() warns (DeprecationWarning; the real
  DisplayManager has none), MockDisplayManager.draw_text accepts the real
  signature's optional params, VisualTestDisplayManager logs draw errors at
  WARNING.
- Docs/comments: compatibility.py method name, PluginState.LOADED meaning,
  brittle schema count, why _report_skip_once uses setdefault.
- Remove unused PluginOperationQueue.get_active_operations().

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

src/plugin_system/compatibility.py

@@ -193,7 +193,7 @@ def declared_min_version(manifest: Dict[str, Any]) -> Optional[str]:
     """The core version this plugin says it needs, or ``None`` if it doesn't say.
 
     Checked in order of specificity. `ledmatrix_min` is the deprecated spelling
-    of `ledmatrix_min_version` (`store_manager._validate_manifest_fields` flags
+    of `ledmatrix_min_version` (`store_manager._validate_manifest_version_fields` flags
     it); both are read because a large share of published manifests still carry
     the old one.