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)

docs/rewrite/notes/ghostty-core.md

@@ -40,7 +40,7 @@ Wiring details: renderer->app via `Host` (scrollbar/health/redraw as app mailbox
 apprt-gtk got `RtSurface::gl_proc_address` (libepoxy `epoxy_*` pointer variables) and `create_scope_fn` (cgroup scope over D-Bus from the IO thread).
 Deviations/stubs: inspector no-op; locale only via GTK's setlocale (no unsafe in core/root); crash reporting (sentry) not ported; `Rust version` row of `+version` empty;
 default DPI 96 (Zig linux).
-How to run: `GSETTINGS_BACKEND=memory GHOSTTY_RESOURCES_DIR=/usr/share/ghostty cargo run -- -e sh -c '...'` (debug builds only find resources via the env var here).
+How to run (resources are found automatically via `<workspace>/resources`, see notes/cleanup-V2.md): `GSETTINGS_BACKEND=memory cargo run -- -e sh -c '...'`.
 Dev screenshot: `GHOSTTY_DEBUG_SCREENSHOT=/abs/out.png [GHOSTTY_DEBUG_SCREENSHOT_MS=2500]` dumps the renderer's last frame (compositor screenshots under niri come out black / are blocked).
 Verified manually: window opens on Wayland and X11, shell prompt and SGR colored output render (target/screenshots/frame.png, shell.png), child exit closes window, clean shutdown.
 NOT verified: keyboard input end-to-end (no way to inject keys into the window here; xdotool/niri did not deliver), mouse, clipboard, IME, splits/tabs.