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/src/file.rs

@@ -101,12 +101,6 @@ pub fn alloc_tmp_dir_os() -> Option<std::path::PathBuf> {
     Some("/tmp".into())
 }
 
-/// Return the recommended path for temporary files. Non-UTF-8 bytes are
-/// carried via [`crate::osbytes`]; see [`alloc_tmp_dir_os`].
-pub fn alloc_tmp_dir() -> Option<String> {
-    alloc_tmp_dir_os().map(|p| crate::osbytes::os_to_config_string(p.as_os_str()))
-}
-
 #[cfg(all(test, unix))]
 mod tests {
     use super::*;
@@ -115,13 +109,13 @@ mod tests {
     #[test]
     fn tmp_dir_env_precedence() {
         temp_env::with_vars([("TMPDIR", Some("/a")), ("TMP", Some("/b"))], || {
-            assert_eq!(alloc_tmp_dir().as_deref(), Some("/a"));
+            assert_eq!(alloc_tmp_dir_os(), Some("/a".into()));
         });
         temp_env::with_vars([("TMPDIR", None), ("TMP", Some("/b"))], || {
-            assert_eq!(alloc_tmp_dir().as_deref(), Some("/b"));
+            assert_eq!(alloc_tmp_dir_os(), Some("/b".into()));
         });
         temp_env::with_vars([("TMPDIR", None::<&str>), ("TMP", None)], || {
-            assert_eq!(alloc_tmp_dir().as_deref(), Some("/tmp"));
+            assert_eq!(alloc_tmp_dir_os(), Some("/tmp".into()));
         });
     }