fix(pty): IsolateCrumbFile creates its parent dir (soak crumb capture)

The T-436 soak run came back green but produced no conpty-kill-crumbs artifact:
IsolateCrumbFile opened its append handle without creating the parent dir, and
the standalone orphan probe points it at a fresh CLIDE_LOG_DIR that nothing
else had made — so openSync failed, the crumb file was disabled, and no crumbs
were written. In the app this was masked because FileLogSink already creates
logDirectory() at boot. Create the parent ourselves (no-op when it exists).
Verified by the bundle-smoke artifact, which DID capture a real watchdog sample
(threads=31, handles=25, rssMB=175) because the release app makes the dir.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/src/pty/pty_log.dart

@@ -65,7 +65,12 @@ class IsolateCrumbFile {
   IsolateCrumbFile(String? path, this.source, {int capBytes = 256 * 1024}) : _capBytes = capBytes {
     if (path == null) return;
     try {
-      final raf = File(path).openSync(mode: FileMode.append);
+      final f = File(path);
+      // Create the parent dir ourselves — a standalone caller (the soak probe)
+      // may point us at a dir nothing else has made yet. In the app the
+      // FileLogSink already created logDirectory(), so this is a no-op there.
+      f.parent.createSync(recursive: true);
+      final raf = f.openSync(mode: FileMode.append);
       _raf = raf;
       _size = raf.lengthSync();
     } catch (_) {

test/pty/pty_log_test.dart

@@ -44,6 +44,14 @@ void main() {
       c.close();
     });
 
+    test('creates a missing parent directory (standalone soak probe case)', () {
+      final nested = '${dir.path}${Platform.pathSeparator}a${Platform.pathSeparator}b${Platform.pathSeparator}crumbs.log';
+      final c = IsolateCrumbFile(nested, 'conpty.reader')..crumb('ReadFile enter');
+      c.close();
+      expect(File(nested).existsSync(), isTrue);
+      expect(File(nested).readAsStringSync(), contains('ReadFile enter'));
+    });
+
     test('writes one tagged, timestamped line per crumb', () {
       final c = IsolateCrumbFile(path(), 'pty.reader');
       expect(c.enabled, isTrue);