fix(testmode): skip the fd-inheritance probe when its helper is absent

The probe spawns a hand-built /tmp/checkfd binary that exists only on
machines where the T-438-era investigation ran; everywhere else it
failed the whole harness with a ProcessException. Absence is now an
explicit skip line, and the helper path is a named constant in both
the io and web-stub variants.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/fd_check_io.dart

@@ -12,6 +12,10 @@ import 'package:ffi/ffi.dart' as pkg_ffi;
 
 import 'src/pty/ffi/libc.dart' as libc;
 
+/// The hand-built helper binary the probe spawns. Not part of the repo — the
+/// harness skips this probe when it is absent.
+const fdCheckHelperPath = '/tmp/checkfd';
+
 /// Probe whether `Process.start` inherits socket fds (the macOS question the
 /// testmode harness answers). Returns a human-readable result line.
 Future<String> fdInheritanceCheck() async {
@@ -20,7 +24,7 @@ Future<String> fdInheritanceCheck() async {
   final parent = sv[0];
   final child = sv[1];
   pkg_ffi.calloc.free(sv);
-  final proc = await Process.start('/tmp/checkfd', [], environment: {...Platform.environment, 'PTYC_SOCK_FD': '$child'});
+  final proc = await Process.start(fdCheckHelperPath, [], environment: {...Platform.environment, 'PTYC_SOCK_FD': '$child'});
   final stderr = await proc.stderr.transform(utf8.decoder).join();
   final exit = await proc.exitCode;
   libc.close(parent);

lib/fd_check_stub.dart

@@ -1,4 +1,7 @@
 /// Web stub (T-438 web fence, D-100): no FFI fd-inheritance probe on web.
 library;
 
+/// Mirrors [fd_check_io.dart]; never exists on web, so the probe is skipped.
+const fdCheckHelperPath = '/tmp/checkfd';
+
 Future<String> fdInheritanceCheck() async => 'skipped (no FFI on web)';

lib/test_app.dart

@@ -381,7 +381,13 @@ class _ClideTestAppState extends State<ClideTestApp> {
 
     // Test: does Dart's Process.start inherit socket fds on macOS? (T-438: the
     // FFI body lives in fd_check_io.dart so the web build can stub it out.)
+    // The probe needs the hand-built /tmp/checkfd helper; on machines without
+    // it, skip explicitly rather than failing the whole harness.
+    if (File(fdCheckHelperPath).existsSync()) {
       await _testAsync('fd inheritance check', fdInheritanceCheck);
+    } else {
+      _say('skip | fd inheritance check | $fdCheckHelperPath not present (hand-built probe helper — see fd_check_io.dart)');
+    }
 
     _say('');
   }