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/main.dart

@@ -48,10 +48,15 @@ Future<void> main() async {
   final binding = WidgetsFlutterBinding.ensureInitialized();
 
   // Test mode: skip the full app, run the test harness instead.
-  const testMode = String.fromEnvironment('CLIDE_TESTMODE');
-  if (testMode.isNotEmpty) {
-    runApp(const ClideTestApp());
-    return;
+  // Gated on kDebugMode so the release tree-shaker can elide both the
+  // branch and the test_app import graph — production binaries don't
+  // ship the harness.
+  if (kDebugMode) {
+    const testMode = String.fromEnvironment('CLIDE_TESTMODE');
+    if (testMode.isNotEmpty) {
+      runApp(const ClideTestApp());
+      return;
+    }
   }
 
   binding.ensureSemantics();