refactor: type escaped OS strings and fix the final audit findings

Escaped config strings (ghostty_os::osbytes) kept reaching OS
boundaries undecoded, and decoding was occasionally applied to plain
text. The root cause was that both were plain `String`s. Now:
- `EncodedOs` is a newtype (no Display/AsRef<Path>) that decodes only
  via to_os/to_path/to_bytes/to_wide; plain text enters via
  from_plain_text. OS helpers (env, home, xdg, passwd, resources dir,
  path expansion), config::Path, WorkingDirectory, theme/file_load/edit
  paths, terminal pwd, OpenUrl/Pwd actions and termio cwd use it, so the
  compiler forces decoding at every boundary. os::open takes &OsStr.
  Config output is unchanged.

Fixes all rows of docs/rewrite/notes/review-rv5-codex.md (final-Z7.md):
screen-file copy; regex/OSC 8 URLs no longer decoded (regression);
lossless Windows WTF-8/WTF-16 for args, env and CreateProcessW; Windows
resources dir/TERMINFO; color OSC request lists and X11 color lookup no
longer allocate globally (C zero-allocation test extended); macOS Mach
handler resumes all threads before KERN_FAILURE (hang regression) and
SIGABRT minidumps go through the exception thread; plus open-config,
background images, crash dir, ssh cache, exe dir/PATH, fontconfig
filenames, file links, kitty TMPDIR, .hushlogin and benchmark paths.
The typing also surfaced and fixed gtk-custom-css, bell audio, bash
HISTFILE, OSC 52 writes and the C config getters.

Test Plan:
- cargo test --workspace (3376 passed)
- just check-macos (both), just check-windows, just check-wasm
- diff <(ghostty-rs +show-config --default --docs) \
  <(ghostty +show-config --default --docs)

crates/ghostty-lib/src/action_c.rs

@@ -131,7 +131,7 @@ pub fn action_to_c(action: &Action, keep: &mut Keep) -> ghostty_action_s {
         Action::SetTabTitle(t) => v.set_tab_title.title = keep.cstr(&t.title),
         Action::PromptTitle(p) => v.prompt_title = *p as c_int,
         Action::Pwd(p) => {
-            v.pwd.pwd = keep.cstr_bytes(&ghostty_os::osbytes::config_str_to_bytes(&p.pwd));
+            v.pwd.pwd = keep.cstr_bytes(&p.pwd.to_bytes());
         }
         Action::MouseShape(s) => v.mouse_shape = *s as c_int,
         Action::MouseVisibility(m) => v.mouse_visibility = *m as c_int,
@@ -185,7 +185,7 @@ pub fn action_to_c(action: &Action, keep: &mut Keep) -> ghostty_action_s {
             v.open_url.kind = u.kind as c_int;
             // The URL may be the escaped form of a non-UTF-8 path; hand the
             // embedder the real bytes.
-            let url = ghostty_os::osbytes::config_str_to_bytes(&u.url);
+            let url = u.url.to_bytes();
             v.open_url.url = keep.cstr_bytes(&url);
             v.open_url.len = url.len();
         }