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/class/split_tree.rs

@@ -940,18 +940,36 @@ mod split_imp {
     #[gtk::template_callbacks]
     impl SplitTreeSplit {
         #[template_callback]
-        fn notify_max_position(&self, _pspec: glib::ParamSpec) {
-            self.obj().refresh();
+        fn notify_max_position(
+            _paned: &gtk::Paned,
+            _pspec: glib::ParamSpec,
+            this: &super::SplitTreeSplit,
+        ) {
+            // The emitter is the child `Paned`; the template instance is the
+            // trailing user-data argument.
+            this.refresh();
         }
 
         #[template_callback]
-        fn notify_min_position(&self, _pspec: glib::ParamSpec) {
-            self.obj().refresh();
+        fn notify_min_position(
+            _paned: &gtk::Paned,
+            _pspec: glib::ParamSpec,
+            this: &super::SplitTreeSplit,
+        ) {
+            // The emitter is the child `Paned`; the template instance is the
+            // trailing user-data argument.
+            this.refresh();
         }
 
         #[template_callback]
-        fn notify_position(&self, _pspec: glib::ParamSpec) {
-            self.obj().refresh();
+        fn notify_position(
+            _paned: &gtk::Paned,
+            _pspec: glib::ParamSpec,
+            this: &super::SplitTreeSplit,
+        ) {
+            // The emitter is the child `Paned`; the template instance is the
+            // trailing user-data argument.
+            this.refresh();
         }
     }