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/post_fork.rs

@@ -1,15 +1,12 @@
 //! Parent post-fork hook creating the transient scope (port of
 //! `apprt/gtk/post_fork.zig`).
 
-use std::time::{Duration, Instant};
-
 use ghostty_config::config::Config;
 use ghostty_config::types_b::{GtkSingleInstance, LinuxCgroup};
 use gtk::gio;
 use gtk::prelude::*;
 
 use crate::cgroup;
-use crate::pre_exec::current_scope_matches;
 
 /// Allowable error from a post-fork hook; fails surface creation.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, thiserror::Error)]
@@ -87,25 +84,16 @@ pub fn post_fork(info: &PostForkInfo, pid: Option<u32>) -> Result<(), PostForkEr
         return Ok(());
     }
 
-    let start = Instant::now();
-    loop {
-        if start.elapsed() > Duration::from_millis(250) {
-            // Zig checks `rt_pre_exec_info.linux_cgroup_hard_fail` here;
-            // both infos come from the same config so they're identical.
-            if info.linux_cgroup_hard_fail {
-                log::error!("transition to new transient systemd scope {expected} took too long");
-                return Err(PostForkError);
-            }
-            log::warn!("transition to transient systemd scope {expected} took too long");
-            break;
+    if ghostty_os::cgroup::wait_for_scope(pid, &expected) {
+        log::debug!("transition to transient systemd scope {expected} complete");
+    } else {
+        // Zig checks `rt_pre_exec_info.linux_cgroup_hard_fail` here;
+        // both infos come from the same config so they're identical.
+        if info.linux_cgroup_hard_fail {
+            log::error!("transition to new transient systemd scope {expected} took too long");
+            return Err(PostForkError);
         }
-
-        if current_scope_matches(pid, &expected) {
-            log::debug!("transition to transient systemd scope {expected} complete");
-            break;
-        }
-
-        std::thread::sleep(Duration::from_millis(25));
+        log::warn!("transition to transient systemd scope {expected} took too long");
     }
 
     Ok(())