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/STATUS.md

@@ -35,7 +35,7 @@ Legend: ⬜ todo · 🟨 in progress · ✅ done (gate passed, committed) · ⚠
 | RV1 | Codex read-only audit core Surface vs Zig | codex astra | ✅ | 9 findings (notes/review-core-codex.md) → handed to V1 |
 | P1 | ghostty-bench port, Zig baseline, perf tuning | sonnet | ✅ | stream at Zig parity; screen-clone 2.6x |
 | V1b | finish V1: clippy, audit fixes, GTK replay check | sonnet | 🟨 | |
-| P2 | perf: screen-clone, osc-parser | sonnet | 🟨 | |
+| P2 | perf: screen-clone, osc-parser | sonnet | ✅ | clone at parity; render 2.4x |
 
 ## Log
 - 2026-10-11: Phase 0 complete; Phase 1 launched.