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-os/tests/flatpak_mock.rs

@@ -53,7 +53,7 @@ fn host_command_spawn_exit_status_and_signal() {
             w,
             devnull(),
         );
-        cmd.cwd = Some("/".into());
+        cmd.cwd = Some(ghostty_os::osbytes::EncodedOs::from_encoded("/"));
         let mut env = ghostty_os::env::EnvMap::default();
         env.insert("FOO", "bar");
         cmd.env = Some(env);
@@ -84,7 +84,7 @@ fn host_command_spawn_exit_status_and_signal() {
             devnull(),
             devnull(),
         );
-        cmd.cwd = Some("/".into());
+        cmd.cwd = Some(ghostty_os::osbytes::EncodedOs::from_encoded("/"));
         cmd.spawn().unwrap();
         let (tx, rx) = std::sync::mpsc::channel();
         cmd.wait_callback(move |r| tx.send(r).unwrap()).unwrap();
@@ -99,7 +99,7 @@ fn host_command_spawn_exit_status_and_signal() {
             devnull(),
             devnull(),
         );
-        cmd.cwd = Some("/".into());
+        cmd.cwd = Some(ghostty_os::osbytes::EncodedOs::from_encoded("/"));
         assert_eq!(cmd.spawn(), Err(Error::FlatpakSpawnFail));
     }
 
@@ -113,7 +113,7 @@ fn host_command_spawn_exit_status_and_signal() {
             devnull(),
             devnull(),
         );
-        cmd.cwd = Some("/".into());
+        cmd.cwd = Some(ghostty_os::osbytes::EncodedOs::from_encoded("/"));
         let pid = cmd.spawn().unwrap();
         let handle = cmd.signal_handle();
         let (tx, rx) = std::sync::mpsc::channel();