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/command.rs

@@ -17,7 +17,6 @@ use std::{
     ffi::{CString, NulError, c_char},
     os::fd::OwnedFd,
     panic::{AssertUnwindSafe, catch_unwind},
-    time::{Duration, Instant},
 };
 
 use ghostty_os::file::Rlimit;
@@ -497,23 +496,7 @@ pub fn fmt_scope(pid: u32) -> String {
     format!("app-ghostty-surface-transient-{pid}.scope")
 }
 
-/// Wait for the current process' cgroup to end in `expected`. Returns true if
-/// seen, false on timeout (250ms).
-fn wait_for_cgroup(pid: u32, expected: &str) -> bool {
-    let start = Instant::now();
-    loop {
-        if start.elapsed() > Duration::from_millis(250) {
-            return false;
-        }
-        if let Some(raw) = ghostty_os::cgroup::current(pid)
-            && let Some(idx) = raw.rfind('/')
-            && &raw[idx + 1..] == expected
-        {
-            return true;
-        }
-        std::thread::sleep(Duration::from_millis(25));
-    }
-}
+use ghostty_os::cgroup::wait_for_scope as wait_for_cgroup;
 
 /// If we are expecting to be moved to a transient systemd scope, wait to see
 /// if that happens by checking for the correct name of the current cgroup.