fix(display): snapshot the plugin mappings under their locks

Addresses the review finding on the cross-thread reads.

_enabled_plugin_not_running runs on the config-watcher thread and read two
mappings the render thread mutates. Catching RuntimeError was not a fix: it
turned a torn read into a coin flip between an unnecessary discovery scan and
a missed retry, which is the bug this PR exists to remove.

Both reads are now snapshots taken under the lock that guards their writes:

- plugin_manifests via a new PluginManager.discovered_plugin_ids(), which
  copies the ids while holding the existing _discovery_lock. Discovery
  rebuilds that mapping entry by entry, so an unsynchronised reader can see
  it half-populated.
- plugin_display_modes under a new controller lock, taken at the only two
  sites that mutate it (_register_loaded_plugin / _unregister_plugin).

The locks are never nested -- each snapshot is taken and released before the
next -- so this cannot deadlock against discovery, which holds _discovery_lock
while it rebuilds.

No cost on the per-frame path. Both mutation sites run during reconcile, which
is rare, and every hot-path read of plugin_display_modes is on the render
thread itself, same thread as the writes, so those stay lock-free.

Tests: the accessor returns a snapshot rather than a live view, and actually
takes the discovery lock (proved from a second thread, since an RLock is
reentrant on the owning one) so a later refactor cannot quietly drop it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01STMbQE4YctTacQXfbYqKuW

test/test_display_controller_plugin_toggle.py

@@ -274,6 +274,7 @@ class TestEnabledPluginNotRunning:
     def test_queues_reconcile_when_enabled_plugin_is_absent(self, test_display_controller):
         controller = test_display_controller
         controller.plugin_manager.plugin_manifests = {"hockey-scoreboard": {}}
+        controller.plugin_manager.discovered_plugin_ids.return_value = {"hockey-scoreboard"}
         controller.plugin_display_modes = {}  # failed to load
         cfg = {"hockey-scoreboard": {"enabled": True, "nhl": {"enabled": True}}}
         assert controller._enabled_plugin_not_running(cfg) is True
@@ -281,6 +282,7 @@ class TestEnabledPluginNotRunning:
     def test_quiet_when_every_enabled_plugin_is_running(self, test_display_controller):
         controller = test_display_controller
         controller.plugin_manager.plugin_manifests = {"hockey-scoreboard": {}}
+        controller.plugin_manager.discovered_plugin_ids.return_value = {"hockey-scoreboard"}
         controller.plugin_display_modes = {"hockey-scoreboard": ["nhl"]}
         cfg = {"hockey-scoreboard": {"enabled": True}}
         assert controller._enabled_plugin_not_running(cfg) is False
@@ -288,6 +290,7 @@ class TestEnabledPluginNotRunning:
     def test_disabled_plugin_does_not_queue(self, test_display_controller):
         controller = test_display_controller
         controller.plugin_manager.plugin_manifests = {"hockey-scoreboard": {}}
+        controller.plugin_manager.discovered_plugin_ids.return_value = {"hockey-scoreboard"}
         controller.plugin_display_modes = {}
         cfg = {"hockey-scoreboard": {"enabled": False}}
         assert controller._enabled_plugin_not_running(cfg) is False
@@ -298,6 +301,7 @@ class TestEnabledPluginNotRunning:
         a reconcile, and therefore a filesystem scan, on every config save."""
         controller = test_display_controller
         controller.plugin_manager.plugin_manifests = {"hockey-scoreboard": {}}
+        controller.plugin_manager.discovered_plugin_ids.return_value = {"hockey-scoreboard"}
         controller.plugin_display_modes = {"hockey-scoreboard": ["nhl"]}
         cfg = {
             "hockey-scoreboard": {"enabled": True},
@@ -309,6 +313,7 @@ class TestEnabledPluginNotRunning:
     def test_non_dict_section_is_ignored(self, test_display_controller):
         controller = test_display_controller
         controller.plugin_manager.plugin_manifests = {"hockey-scoreboard": {}}
+        controller.plugin_manager.discovered_plugin_ids.return_value = {"hockey-scoreboard"}
         controller.plugin_display_modes = {}
         assert controller._enabled_plugin_not_running({"hockey-scoreboard": "nonsense"}) is False
 
@@ -347,6 +352,7 @@ class TestReconcileQueuedThroughSubscriber:
     def test_nested_edit_queues_reconcile_for_absent_plugin(self, test_display_controller):
         controller = test_display_controller
         controller.plugin_manager.plugin_manifests = {"hockey-scoreboard": {}}
+        controller.plugin_manager.discovered_plugin_ids.return_value = {"hockey-scoreboard"}
         controller.plugin_display_modes = {}          # validate_config() said False
         controller._pending_plugin_reconcile = False
 
@@ -367,6 +373,7 @@ class TestReconcileQueuedThroughSubscriber:
         render thread."""
         controller = test_display_controller
         controller.plugin_manager.plugin_manifests = {"hockey-scoreboard": {}}
+        controller.plugin_manager.discovered_plugin_ids.return_value = {"hockey-scoreboard"}
         controller.plugin_display_modes = {"hockey-scoreboard": ["nhl"]}
         controller._pending_plugin_reconcile = False