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>

test/test_plugin_loader.py

@@ -342,3 +342,49 @@ class TestPluginLoader:
 
         assert result is False
         mock_subprocess.assert_not_called()
+
+    @patch('src.plugin_system.plugin_loader.requirements_are_satisfied', return_value=False)
+    def test_install_dependencies_never_runs_pip_concurrently(
+        self, mock_satisfied, tmp_plugins_dir
+    ):
+        """Startup loads plugins on a thread pool; two pip processes writing
+        the same site-packages at once can corrupt it, so installs for
+        different plugins must run one at a time."""
+        import threading
+        import time
+
+        state = {'running': 0, 'peak': 0}
+        guard = threading.Lock()
+
+        def fake_pip(*args, **kwargs):
+            with guard:
+                state['running'] += 1
+                state['peak'] = max(state['peak'], state['running'])
+            time.sleep(0.05)
+            with guard:
+                state['running'] -= 1
+            return MagicMock(returncode=0, stderr="")
+
+        plugin_dirs = []
+        for n in range(4):
+            d = tmp_plugins_dir / f"plugin_{n}"
+            d.mkdir()
+            (d / "requirements.txt").write_text("package1==1.0.0\n")
+            plugin_dirs.append(d)
+
+        results = []
+
+        def install(d):
+            results.append(PluginLoader().install_dependencies(
+                d, d.name, plugins_dir=tmp_plugins_dir))
+
+        with patch('subprocess.run', side_effect=fake_pip) as mock_subprocess:
+            threads = [threading.Thread(target=install, args=(d,)) for d in plugin_dirs]
+            for t in threads:
+                t.start()
+            for t in threads:
+                t.join()
+
+        assert results == [True] * 4
+        assert mock_subprocess.call_count == 4
+        assert state['peak'] == 1