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-cli/src/validate_config.rs

@@ -3,6 +3,7 @@
 use std::io::Write;
 
 use ghostty_config::{config::Config, config_struct};
+use ghostty_os::osbytes::EncodedOs;
 
 use crate::error::Error;
 
@@ -41,8 +42,8 @@ fn run_inner(opts: &Options, stdout: &mut dyn Write) -> Result<u8, Error> {
     if let Some(config_path) = &opts.config_file {
         // Byte-preserving: the option string carries non-UTF-8 bytes via the
         // reversible `osbytes` escape and the canonical path may too.
-        let abs_path = std::fs::canonicalize(ghostty_os::osbytes::config_str_to_path(config_path))?;
-        cfg.load_file(&ghostty_os::osbytes::path_to_config_string(&abs_path))?;
+        let abs_path = std::fs::canonicalize(EncodedOs::from_encoded(config_path).to_path())?;
+        cfg.load_file(&EncodedOs::from_path(&abs_path))?;
         cfg.load_recursive_files()?;
     } else {
         cfg = Config::load()?;