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/env.dart

@@ -0,0 +1,35 @@
+/// Default environment for PTY-spawned children.
+///
+/// `xterm.dart` on the UI side + most shells + tmux + Claude CLI all
+/// understand the 24-bit-colour triplet `TERM=xterm-256color` +
+/// `COLORTERM=truecolor`. Without `COLORTERM` most apps fall back to
+/// the 256-colour palette and the terminal looks washed out even though
+/// the renderer can do true colour.
+library;
+
+/// Base env clide's daemon builds for every PTY child. Callers merge
+/// with the user's environment before passing to `ptyc` — a child that
+/// needs user env like `HOME` / `USER` / `SHELL` still gets them; the
+/// keys here override the ones the child cares about.
+const Map<String, String> clidePtyEnvDefaults = {
+  'TERM': 'xterm-256color',
+  'COLORTERM': 'truecolor',
+  // Encourages 24-bit emission from tooling that checks this:
+  'CLICOLOR_FORCE': '1',
+  // tmux inherits these when clide spawns tmux; safe to propagate.
+  'LANG': 'en_US.UTF-8',
+  'LC_ALL': 'en_US.UTF-8',
+};
+
+/// Merge [base] onto the process environment; clide defaults override
+/// user env where they overlap. Explicit [overrides] win over both.
+Map<String, String> mergePtyEnv({
+  required Map<String, String> processEnv,
+  Map<String, String>? overrides,
+}) {
+  return {
+    ...processEnv,
+    ...clidePtyEnvDefaults,
+    if (overrides != null) ...overrides,
+  };
+}