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-apprt-gtk/src/pre_exec.rs

@@ -1,9 +1,8 @@
 //! Child pre-exec hook for cgroup transitions (port of `apprt/gtk/pre_exec.zig`).
 //!
 //! Runs in the child between fork and exec, so it only reads procfs and
-//! sleeps. The logic is also mirrored by `ghostty-termio`'s `Command`.
-
-use std::time::{Duration, Instant};
+//! sleeps. The wait loop is shared with `ghostty-termio` via
+//! `ghostty_os::cgroup::wait_for_scope`.
 
 use ghostty_config::config::Config;
 use ghostty_config::types_b::{GtkSingleInstance, LinuxCgroup};
@@ -46,36 +45,13 @@ pub fn pre_exec(info: &PreExecInfo) -> Option<u8> {
 
     let pid = std::process::id();
     let expected = cgroup::fmt_scope(pid);
-    let start = Instant::now();
-
-    loop {
-        if start.elapsed() > Duration::from_millis(250) {
-            if info.linux_cgroup_hard_fail {
-                log::error!("transition to new transient systemd scope took too long");
-                return Some(127);
-            }
-            break;
-        }
-
-        if current_scope_matches(pid, &expected) {
-            return None;
-        }
-
-        std::thread::sleep(Duration::from_millis(25));
+    if !ghostty_os::cgroup::wait_for_scope(pid, &expected) && info.linux_cgroup_hard_fail {
+        log::error!("transition to new transient systemd scope took too long");
+        return Some(127);
     }
-
     None
 }
 
-/// True if the final path component of `pid`'s cgroup equals `expected`.
-pub(crate) fn current_scope_matches(pid: u32, expected: &str) -> bool {
-    let Some(raw) = ghostty_os::cgroup::current(pid) else {
-        return false;
-    };
-    raw.rsplit_once('/')
-        .is_some_and(|(_, last)| last == expected)
-}
-
 #[cfg(test)]
 mod tests {
     use super::*;