refactor(plugins): one plugin-directory resolver (#623)

* refactor(plugins): one resolver for plugin id -> directory

Five places mapped a plugin id to its directory, each with its own rules
and each re-reading manifests per lookup: PluginManager discovery and
get_plugin_directory, PluginLoader.find_plugin_directory,
PluginStoreManager._find_plugin_path / list_installed_plugins, and
state_reconciliation.disk_plugin_ids. They disagreed on backup dirs,
on whether the manifest id or the directory name is the id, on duplicate
ids and on path safety.

src/plugin_system/plugin_dirs.py now holds the rules once:
PluginDirectoryIndex scans one directory and reads each manifest once;
resolve_plugin_dir() searches directories in order. What legitimately
differs per caller is an explicit argument: search dirs (discovery and
the loader: configured dir only; the store: configured then sibling
plugins/), ledmatrix- prefix (not for the store), case folding (loader
only), manifest pass (not for get_plugin_directory, whose discovery map
already holds it).

Behaviour changes, all for layouts installs do not produce:
- a directory whose manifest declares the id beats one merely named for
  it (discovery already worked this way; the loader and store now agree)
- the store searches the configured dir completely before plugins/
- backup and hidden dirs are skipped everywhere (the loader's case and
  manifest scans and list_installed_plugins used to return them)
- duplicate ids resolve deterministically (exact name, then
  ledmatrix-<id>, then by name) with a one-time warning; discovery no
  longer lists the id twice
- disk_plugin_ids / list_installed_plugins report manifest ids, falling
  back to the directory name; auto-update looks the directory up
- ids that are not one plain path segment resolve to nothing in every
  caller (the loader used to truncate them, the store to join them)

The .standalone-backup- marker is one constant, BACKUP_MARKER, used by
store_manager's rename-aside names and every lookup.

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

* docs(changelog): one plugin-directory resolver

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

---------

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

web_interface/auto_update.py

@@ -172,9 +172,10 @@ def _plugin_fingerprint(store_manager, plugin_dir):
     try:
         with open(plugin_dir / 'manifest.json', 'r', encoding='utf-8') as f:
             manifest = json.load(f)
-        if manifest.get('local_only'):
-            return None
-        version = manifest.get('version')
+        if isinstance(manifest, dict):  # valid JSON need not be an object
+            if manifest.get('local_only'):
+                return None
+            version = manifest.get('version')
     except (OSError, ValueError):
         pass
     sha = None
@@ -190,8 +191,12 @@ def update_plugins(store_manager, operation_history=None):
     """Update every installed plugin that has an update. Returns (updated, failed)."""
     updated, failed = [], []
     plugins_dir = Path(store_manager.plugins_dir)
+    # list_installed_plugins() reports manifest ids, and a plugin's directory
+    # may be named differently (ledmatrix-stocks/ holding id "stocks"), so
+    # the directory comes from the store's own lookup, not a join.
+    find_dir = getattr(store_manager, '_find_plugin_path', None)
     for plugin_id in sorted(store_manager.list_installed_plugins()):
-        plugin_dir = plugins_dir / plugin_id
+        plugin_dir = (find_dir(plugin_id) if find_dir else None) or plugins_dir / plugin_id
         before = _plugin_fingerprint(store_manager, plugin_dir)
         if before is None:
             continue  # local_only: managed by hand, never from the registry