test(core): add headless end-to-end tests and fix bugs they found

Adds crates/ghostty-core/tests/e2e_*.rs: 81 tests that drive the real
core Surface, termio, pty and child processes through a fake runtime
(recording apprt actions, in-memory clipboard) with a headless renderer
(ghostty-renderer/src/headless.rs). They cover keys (legacy, DECCKM,
kitty, modifyOtherKeys), keybinding actions and sequences, mouse
selection/reporting/scrolling/links, clipboard incl. OSC 52 and paste
safety, resize, focus, titles/pwd/bell/notifications/progress, child
exit, and IME preedit.

Fixes for issues found by the tests and by the Codex audit
(docs/rewrite/notes/review-core-codex.md, all 9 items):
- replacing a selection with copy-on-select panicked (stale tracked
  handles); global new_window no longer inherits a parent; the child
  environment uses the runtime's env wholesale so GTK's removals stick;
  Flatpak host resources path; locale recovery (ensureLocale) in
  apprt-gtk before GTK init and for CLI actions; failed screen-file
  opens propagate errors; OSC 52 base64 rules, CSI/ESC binding length
  limits and GHOSTTY_LOG parsing match Zig.
- Also includes V2's termio cleanups (tmux viewer wiring in the stream
  handler, Pty slave semantics docs, shared cgroup wait loop).

apprt-gtk gains a debug input replay hook (GHOSTTY_DEBUG_INPUT_SCRIPT)
exercising the real GTK key path. With it, the real app was verified on
Wayland: typing and output, new tab, splits, select-all/copy/paste and
font size all work (docs/rewrite/notes/verification-V1.md). Not yet
exercised at GTK level: mouse, real IM-context IME, clipboard dialogs.

Test Plan:
- cargo test --workspace (2983 passed)
- cargo test -p ghostty-core --tests (81 e2e tests)

crates/ghostty-termio/src/pty.rs

@@ -92,8 +92,13 @@ pub enum Error {
 /// A pty master/slave pair.
 ///
 /// Deviation from Zig: both fds are `OwnedFd` and are closed on drop (the Zig
-/// struct never closes the slave automatically). To close the slave early
-/// (as Exec does after the fork) use [`Pty::close_slave`] or destructure.
+/// struct never closes the slave automatically). This is behaviour-preserving:
+/// Zig's `Exec.Subprocess.start` closes the slave explicitly in exactly two
+/// places (in the parent right after the child was started successfully, and
+/// in the start error path before `pty.deinit()`), and `Pty.deinit` is only
+/// reached afterwards. The Rust `Exec` calls [`Pty::close_slave`] after a
+/// successful start and drops the whole `Pty` on error, so the slave fd lives
+/// exactly as long as in Zig.
 #[derive(Debug)]
 pub struct Pty {
     pub master: OwnedFd,