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>

pubspec.yaml

@@ -28,7 +28,13 @@ environment:
 # "write it yourself" default (see memory:
 # feedback_dart_deps_minimal_locked_cve_checked.md). Exact-pin; never
 # carets.
-dependencies: {}
+dependencies:
+  # Dart-team-maintained allocator (`calloc`) + type helpers on top of
+  # dart:ffi. Saves ~50 LOC of malloc/free boilerplate for the PTY
+  # wrapper's libc bindings (socketpair, recvmsg with SCM_RIGHTS,
+  # ioctl). Tiny, zero transitive deps. Listed in
+  # app/assets/licenses.yaml per D-042.
+  ffi: 2.1.3
 
 dev_dependencies:
   lints: 5.0.0