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>

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);