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>

test/test_discovery_path_contract.py

@@ -16,9 +16,13 @@ change to the fallback chains is a deliberate one.
 
 The `.standalone-backup-` contract: store_manager renames a plugin dir aside
 with that substring during install/rollback; discovery MUST skip such dirs
-or a half-finished install would surface a ghost plugin. The substring is
-duplicated as a literal in both files — this test breaks if either side
-changes it unilaterally.
+or a half-finished install would surface a ghost plugin. The substring now
+lives once, as plugin_dirs.BACKUP_MARKER, which both sides import; its value
+is pinned because debris already on devices carries exactly that text.
+
+All of them now resolve through src/plugin_system/plugin_dirs.py; the
+per-caller differences pinned here are explicit arguments there. The rules
+themselves are covered table-style in test_plugin_dirs.py.
 """
 
 import json
@@ -198,14 +202,16 @@ class TestStandaloneBackupContract:
         found = _scanner()._scan_directory_for_plugins(plugins_dir)
         assert found == ["real-plugin"]
 
-    def test_backup_substring_literal_matches_across_files(self):
-        """The substring is duplicated in plugin_manager (skip check) and
-        store_manager (rename-aside names). If either side changes it, the
-        other silently stops honoring the contract — this test is the
-        tripwire."""
+    def test_backup_marker_is_shared_and_unchanged(self):
+        """store_manager (rename-aside names) and every lookup (skip check)
+        must agree on the marker. Both now import one constant; the value is
+        pinned because renaming it would make existing debris on devices
+        visible as plugins again."""
+        from src.plugin_system import plugin_dirs
+        assert plugin_dirs.BACKUP_MARKER == '.standalone-backup-'
         root = Path(__file__).resolve().parents[1]
-        pm_text = (root / "src/plugin_system/plugin_manager.py").read_text()
-        sm_text = (root / "src/plugin_system/store_manager.py").read_text()
-        assert "'.standalone-backup-'" in pm_text.replace('"', "'")
-        assert ".standalone-backup-" in sm_text
-
+        sm_text = (root / "src/plugin_system/store_manager.py").read_text(encoding="utf-8")
+        assert "{BACKUP_MARKER}preinstall" in sm_text
+        assert "{BACKUP_MARKER}migrating" in sm_text
+        assert plugin_dirs.is_ignored_dir_name(
+            "demo" + plugin_dirs.BACKUP_MARKER + "preinstall")