feat(pty): FFI breadcrumbs around the syscalls that wedge (T-434)

The freeze hypothesis is a wedged FFI call — a reader isolate blocked forever
in ReadFile, a waiter in WaitForSingleObject, Isolate.kill unable to interrupt
either. To NAME the wedge after a power-cycle, each backend now drops a
breadcrumb before/after every risky syscall.

- pty_log.dart (new, Flutter-free, tested): PtyLog — an injectable, no-op-by-
  default breadcrumb hook for the MAIN isolate (wired to the kernel Logger,
  source 'conpty'/'pty' = an eager FileLogSink source) — and IsolateCrumbFile,
  which the SPAWNED reader/waiter isolates use to open their OWN append handle
  and flushSync per line, so a wedged isolate's last crumb survives even a
  frozen main isolate (the whole point). Bounded by a truncating size cap.
- native_pty.dart + windows_pty.dart: crumbs around posix_spawn/read and
  CreatePseudoConsole/CreateProcessW/ReadFile/WaitForSingleObject; the reader/
  waiter isolates carry a sendable crumb path + verbose flag. Per-syscall crumbs
  only at debug/trace; lifecycle crumbs always.
- Wiring: startPtySession → PaneRegistry → buildDispatcher build the PtyLog from
  the kernel Logger + a crumb file under logDirectory(); verbose follows the log
  level. Default everywhere is PtyLog.none — zero behaviour change off the wire.

Tested: PtyLog/IsolateCrumbFile units (cap-truncation, append, no-op) + an
end-to-end real-PTY test asserting the reader isolate writes its own crumbs
('reader started' / 'read -> n=' / 'reader exiting'), which validates the
identical Windows structure that can't run here. Coverage gate 95.10%.

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

lib/src/pty/pty_session.dart

@@ -12,6 +12,7 @@ import 'dart:io' show Platform;
 import 'dart:typed_data';
 
 import 'native_pty.dart';
+import 'pty_log.dart';
 import 'windows_pty.dart';
 
 abstract interface class PtySession {
@@ -49,6 +50,7 @@ PtySession startPtySession({
   required int rows,
   String? workingDirectory,
   Map<String, String> environment = const {},
+  PtyLog log = PtyLog.none,
 }) {
   if (Platform.isWindows) {
     return WindowsPty.start(
@@ -58,6 +60,7 @@ PtySession startPtySession({
       rows: rows,
       workingDirectory: workingDirectory,
       environment: environment,
+      log: log,
     );
   }
   return NativePty.start(
@@ -67,5 +70,6 @@ PtySession startPtySession({
     rows: rows,
     workingDirectory: workingDirectory,
     environment: environment,
+    log: log,
   );
 }