add Dart PTY wrapper and test-core harness

PtySession wraps the ptyc helper: socketpair + Process.start + recvmsg
with SCM_RIGHTS for master-fd transfer, a background isolate that
loops on blocking read() and posts byte chunks, plus write/resize/
kill/close. close() SIGTERMs the child so the PTY's EOF wakes the
reader naturally; SIGKILL + fd close + isolate kill cover the edge
where the shell ignores SIGTERM — avoids the known Linux quirk where
closing an fd doesn't unblock an in-flight read() on it.

Env defaults stamp TERM=xterm-256color, COLORTERM=truecolor,
CLICOLOR_FORCE=1 so shells + tmux + Claude emit 24-bit sequences
that xterm.dart can render. User env (HOME / USER / SHELL) still
inherits via mergePtyEnv().

ffi: 2.1.3 added as a runtime dep — the FFI bindings for socketpair,
recvmsg, read/write, and ioctl(TIOCSWINSZ) need an allocator we're
not writing by hand. Justified in pubspec + listed in licenses.yaml
per D-042.

make test-core (ci/test_core.sh) runs the Flutter-free core tests
under a 120s hard timeout with setsid + process-group kill, wired
ahead of the fast app tests in push-check so a hung PTY test can't
wedge a pre-push. Current core suite: 24 tests in ~1s.

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

lib/src/pty/errors.dart

@@ -0,0 +1,23 @@
+/// Error type for the PTY subsystem. Flutter-free — `dart:io`'s
+/// `OSError` + `ProcessException` don't quite fit (we're a mix of
+/// syscall-level and subprocess-level failures), and we can't pull
+/// `PlatformException` from `package:flutter/services.dart` since the
+/// core library stays Flutter-free per D-005.
+library;
+
+/// A PTY operation failed. [op] identifies the step (`recvmsg`,
+/// `socketpair`, `ptyc`, etc.); [errno] is POSIX errno when the
+/// failure came from a syscall, otherwise `null`.
+class PtyException implements Exception {
+  const PtyException(this.op, this.message, {this.errno});
+
+  final String op;
+  final String message;
+  final int? errno;
+
+  @override
+  String toString() {
+    final suffix = errno != null ? ' (errno=$errno)' : '';
+    return 'PtyException($op): $message$suffix';
+  }
+}