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/flatpak.rs

@@ -1,6 +1,6 @@
 //! Flatpak helpers (port of `apprt/gtk/flatpak.zig`).
 
-use ghostty_os::ResourcesDir;
+use ghostty_os::{ResourcesDir, osbytes::EncodedOs};
 use gtk::glib;
 
 /// Returns the resources directory, consulting the Flatpak runtime data
@@ -9,7 +9,7 @@ pub fn resources_dir() -> std::io::Result<ResourcesDir> {
     // Only consult Flatpak runtime data for the host case.
     if ghostty_os::is_flatpak() {
         let mut result = ResourcesDir {
-            app_path: Some("/app/share/ghostty".to_owned()),
+            app_path: Some(EncodedOs::from_plain_text("/app/share/ghostty")),
             host_path: None,
         };
 
@@ -24,13 +24,11 @@ pub fn resources_dir() -> std::io::Result<ResourcesDir> {
             return Ok(result);
         };
 
-        result.host_path = Some(
-            std::path::Path::new(app_dir.as_str())
+        result.host_path = Some(EncodedOs::from_path(
+            &std::path::Path::new(app_dir.as_str())
                 .join("share")
-                .join("ghostty")
-                .to_string_lossy()
-                .into_owned(),
-        );
+                .join("ghostty"),
+        ));
         return Ok(result);
     }