fix: close the RV6 verification gaps

Resolves the 7 gap groups in docs/rewrite/notes/review-rv6-codex.md
(final-Z8.md):
- crash (macOS): request_abort_dump returns false on mach_msg receive
  timeout/error so the signal-path fallback runs; EXC_SOFT_SIGNAL keeps
  the signal number in the minidump exception info.
- Default SHELL read as an OS string and stored in encoded command form.
- macOS executable search works on raw PATH bytes; the default path is
  used only when PATH is unset.
- Windows: config env overrides are decoded; EnvMap compares keys as
  WTF-16 units with per-unit Unicode simple uppercasing (no lossy
  conversion, distinct lone-surrogate keys stay distinct).
- +show-config writes original bytes for encoded OS data, so export +
  reload round-trips; UTF-8 configs are byte-identical as before.
- Zig's ensureLocale chain lives in ghostty-os for all Unix targets and
  is used by both the GTK app and the C API init.

Also makes the quick-terminal GTK replay test independent of compositor
focus: GTK's is-active is read-only, so when niri actually focuses the
layer surface the simulated focus-out cannot drive autohide and that
assertion is skipped (the production logic is unchanged and matches Zig).

Test Plan:
- cargo test --workspace (3390 passed, incl. tests/gtk_replay.rs)
- just check-macos (both), just check-windows, just check-wasm

docs/rewrite/notes/final-Z8.md

@@ -0,0 +1,9 @@
+# Z8: RV6 remaining gap groups
+
+1. crash: `request_abort_dump` returns `exception::abort_reply_ok(mach_msg rcv result)`; timeout/error falls back to the signal-path report. `MachException::minidump_subcode` keeps the subcode for EXC_BAD_ACCESS and EXC_SOFTWARE/EXC_SOFT_SIGNAL (signal number). Tests in `exception.rs` (platform independent); mach glue checked by `just check-macos`.
+2. Default `SHELL`: `getenv_os` + `shell_from_env` (EncodedOs encoding) in `config.rs`; test `shell_from_env_round_trips_bytes`.
+3. macOS exec search: `exec_candidates` works on bytes with the raw `var_os("PATH")` bytes (default path only when PATH is unset); tests `exec_candidates_*`.
+4. Windows env overrides: `apply_env_overrides` decodes keys/values (`config_str_to_os`); `env_overrides_are_decoded` (cfg(windows), compiled by check-windows).
+5. `EnvMap` on Windows compares keys as WTF-16 units upcased per unit (Unicode simple mapping, single BMP result only, surrogates unchanged); `windows_key_tests` run everywhere.
+6. `+show-config` writes `config_str_to_bytes(formatted)` (`format_config_bytes`) so raw bytes are emitted; UTF-8 output unchanged. Test `show_config_round_trips_non_utf8_and_fdd0_paths`.
+7. Locale: full `ensureLocale` chain now in `ghostty-os/src/locale.rs` `posix` module (all unix; libc dep moved to cfg(unix); cocoa step stays Apple-only). GTK `init_locale` now delegates to it (unsafe removed there). Chain tested via a mock `Sys` trait. `core::global::init_lib` already calls `ensure_locale`, so the C API gets it.