replace forkpty() with posix_openpt() + posix_spawn() (T-96)

`forkpty` calls `fork()` underneath. `fork()` in a multithreaded
process is unsafe: only the calling thread survives in the child,
but libc locks held by other threads remain "locked forever." With
the multi-threaded Dart VM as parent, ~5% of spawns deadlocked in
the child before `execve` (forensic probe: child stuck in S state
with comm=`DartWorker`, master fd never sees POLLIN).

`posix_spawn` uses `vfork` on glibc/musl/macOS, keeping the parent
suspended until execve completes — no Dart code runs in the child.
Pty pair built via the POSIX-standard `posix_openpt` / `grantpt` /
`unlockpt` / `ptsname` sequence. Probed: zero hangs in 300
sequential spawns vs ~5% before.

Behavior change: missing executable / missing workingDirectory now
surface as a `PtyException` thrown by `NativePty.start` rather than
a diagnostic written from the child to the slave PTY. Cleaner error
path for callers.

Side benefit: drops the `libutil.so.1` dynamic-library dependency.
PTY now resolves entirely against libc via `DynamicLibrary.process()`.

Splits the library-level `@Tags(['forkpty'])` on session_test.dart
into a per-test tag, so the now-runnable-under-flutter-test cases
contribute to coverage. `dart_test.yaml` declares the tag so the
exclude-tags filters honor it. Drops the `retry: 2` workaround from
the formerly-flaky registry test.

D-5 amended. Trims session-introduced CHANGELOG entries that were
over-verbose for the Keep-a-Changelog format.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/src/pty/pty.dart

@@ -1,6 +1,7 @@
-/// PTY subsystem — spawn child processes under a PTY via forkpty(),
-/// expose their master fd as a byte stream. Desktop IDE's pane model
-/// (terminal / Claude / future tmux wrappers) rides on this.
+/// PTY subsystem — spawn child processes under a PTY via posix_openpt()
+/// + posix_spawn(), expose their master fd as a byte stream. Desktop
+/// IDE's pane model (terminal / Claude / future tmux wrappers) rides on
+/// this.
 library;
 
 export 'env.dart' show clidePtyEnvDefaults, mergePtyEnv;