code-quality batch (T-112)

Seven small consultant findings, one commit:

1. TreeSitterLib stores last dlopen error + path in static fields
   instead of swallowing them. Callers that observe a null instance
   can now read the diagnostic.

2. Drop the Cmsghdr alias in libc.dart — back-compat shim with no
   callers; CLAUDE.md forbids those in a solo repo.

3. Drop EditorController._events field + the unused_field
   suppression. The constructor still subscribes via `events.on<...>`
   for _eventSub; the field itself was speculative retention.

4. Replace inline hex / errno literals in native_pty.dart with
   PosixErrno.{eintr,ebadf,epipe} and new libc.{pollin, pollAnyErr,
   sighup, sigkill, sigwinch}. PosixErrno gains eintr.

5. ExtensionManager records activate/deactivate exceptions in a
   `_failed` map exposed as `failedExtensions` + `didFail(id)`.
   Listeners are notified on entry/exit; cleared on a clean
   activate. UI surfaces the degraded state instead of pretending
   everything is fine.

6. file_tree_view imports FileEntry via the clide.dart barrel
   instead of `package:clide/src/files/listing.dart` directly — the
   leak the consultant flagged (barrel already re-exports it).

7. test_app branch in main.dart wrapped in `if (kDebugMode)` so
   release tree-shaker elides the test harness from shipping
   binaries. Source import stays; tree-shake handles the rest.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/kernel/src/extensions_manager.dart

@@ -81,6 +81,9 @@ class ExtensionManager extends ChangeNotifier {
 
   final Map<String, ClideExtension> _known = {};
   final Set<String> _activated = {};
+  // Extensions whose activate() / deactivate() threw — exposed so the
+  // UI can show a "degraded" status next to them.
+  final Map<String, Object> _failed = {};
 
   void register(ClideExtension ext) {
     if (_known.containsKey(ext.id)) {
@@ -94,6 +97,12 @@ class ExtensionManager extends ChangeNotifier {
   Iterable<ClideExtension> get all => _known.values;
   bool isActivated(String id) => _activated.contains(id);
 
+  /// Map of extension id → most recent activate/deactivate error.
+  /// Cleared for an extension when it activates cleanly. UI surfaces
+  /// read this for the "degraded" indicator.
+  Map<String, Object> get failedExtensions => Map.unmodifiable(_failed);
+  bool didFail(String id) => _failed.containsKey(id);
+
   bool isEnabled(String id) {
     final v = settings.get<bool>('app.extensions.$id.enabled');
     return v ?? true;
@@ -138,11 +147,14 @@ class ExtensionManager extends ChangeNotifier {
         _applyContribution(c);
       }
       _activated.add(id);
+      _failed.remove(id);
       events.emit(ExtensionActivated(id: id));
       notifyListeners();
       log.info('extensions', 'activated $id');
     } catch (e, st) {
+      _failed[id] = e;
       log.error('extensions', 'activate failed for $id', error: e, stackTrace: st);
+      notifyListeners();
     }
   }
 
@@ -160,7 +172,9 @@ class ExtensionManager extends ChangeNotifier {
       notifyListeners();
       log.info('extensions', 'deactivated $id');
     } catch (e, st) {
+      _failed[id] = e;
       log.error('extensions', 'deactivate failed for $id', error: e, stackTrace: st);
+      notifyListeners();
     }
   }