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-apprt-gtk/src/css.rs

@@ -110,29 +110,30 @@ impl CssProviders {
 
         for p in &config.gtk_custom_css.value {
             let (path, optional) = match p {
-                Path::Optional(path) => (path, true),
-                Path::Required(path) => (path, false),
+                Path::Optional(path) => (path.to_path(), true),
+                Path::Required(path) => (path.to_path(), false),
             };
-            let meta = match std::fs::metadata(path) {
+            let shown = path.display();
+            let meta = match std::fs::metadata(&path) {
                 Ok(m) => m,
                 Err(e) => {
                     if e.kind() != std::io::ErrorKind::NotFound || !optional {
-                        log::warn!("error opening gtk-custom-css file {path}: {e}");
+                        log::warn!("error opening gtk-custom-css file {shown}: {e}");
                     }
                     continue;
                 }
             };
-            log::info!("loading gtk-custom-css path={path}");
+            log::info!("loading gtk-custom-css path={shown}");
             if meta.len() > CSS_FILE_SIZE_LIMIT {
                 log::warn!(
-                    "gtk-custom-css file {path} was larger than {CSS_FILE_SIZE_LIMIT} bytes"
+                    "gtk-custom-css file {shown} was larger than {CSS_FILE_SIZE_LIMIT} bytes"
                 );
                 continue;
             }
-            let contents = match std::fs::read(path) {
+            let contents = match std::fs::read(&path) {
                 Ok(c) => c,
                 Err(e) => {
-                    log::warn!("error reading gtk-custom-css file {path}: {e}");
+                    log::warn!("error reading gtk-custom-css file {shown}: {e}");
                     continue;
                 }
             };