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/clide.dart

@@ -4,8 +4,8 @@
 /// under `app/` (which depends on this package via `path: ../`).
 ///
 /// See:
-///   * docs/ADRs/0005-dart-core-ptyc-peer.md — layout + language rationale.
-///   * docs/ADRs/0006-cli-and-event-surface.md — CLI + event contract.
+///   * `decisions/architecture.md` `D-005` — layout + language rationale.
+///   * `decisions/architecture.md` `D-006` — CLI + event contract.
 library;
 
 export 'src/daemon/dispatcher.dart';
@@ -13,6 +13,8 @@ export 'src/ipc/envelope.dart';
 export 'src/ipc/paths.dart';
 export 'src/ipc/schema_v1.dart';
 export 'src/ipc/server.dart';
+export 'src/pty/errors.dart' show PtyException;
+export 'src/pty/pty.dart';
 
 /// Build-time-stamped version string.
 ///