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-core/tests/common/mod.rs

@@ -50,7 +50,7 @@ impl Waker for NopWaker {
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct ClipboardWrite {
     pub clipboard: Clipboard,
-    pub contents: Vec<(String, String)>,
+    pub contents: Vec<(String, Vec<u8>)>,
     pub confirm: bool,
 }
 
@@ -131,13 +131,15 @@ impl RtSurface for FakeRt {
         self.pending.borrow_mut().push_back((clipboard, state));
         true
     }
-    fn set_clipboard(&self, clipboard: Clipboard, contents: &[(String, String)], confirm: bool) {
+    fn set_clipboard(&self, clipboard: Clipboard, contents: &[(String, Vec<u8>)], confirm: bool) {
         self.writes.borrow_mut().push(ClipboardWrite {
             clipboard,
             contents: contents.to_vec(),
             confirm,
         });
-        if let Some((_, text)) = contents.iter().find(|(m, _)| m == "text/plain") {
+        if let Some((_, bytes)) = contents.iter().find(|(m, _)| m == "text/plain") {
+            // Test harness: the fake clipboard stores text.
+            let text = &String::from_utf8_lossy(bytes).into_owned();
             // Like GTK, a confirm=true write shows a dialog first; the
             // harness only stores unconfirmed writes.
             if confirm {