fix: last RV11 gaps - bench enum args, theme NUL paths, Zig error names

Resolves the 3 remaining gaps in docs/rewrite/notes/review-rv11-codex.md
(final-Z14.md):
- ghostty-bench validates every occurrence of enum options in order
  (Args::take_enum; empty value resets) like cli/args.zig, so
  `--mode=bad --mode=noop` fails as in Zig.
- Theme paths with an interior NUL open the prefix like Zig's C-string
  path handling (verified against /usr/bin/ghostty).
- Filesystem diagnostics print Zig's error names (error.FileNotFound,
  NotDir, AccessDenied, IsDir, NameTooLong, SymLinkLoop, ...) via a
  shared ghostty_os::zig_error mapping used by theme/path/config-file/
  file_load/edit-config/list-themes; messages match the oracle.

Test Plan:
- cargo test --workspace --no-fail-fast
- just check-macos (both), just check-windows, just check-wasm

crates/ghostty-cli/src/list_themes/preview.rs

@@ -1396,11 +1396,9 @@ impl Preview {
 fn error_name(e: &ghostty_config::config::Error) -> String {
     use ghostty_config::config::Error;
     match e {
-        Error::Io(io) => match io.kind() {
-            std::io::ErrorKind::NotFound => "FileNotFound".to_string(),
-            std::io::ErrorKind::PermissionDenied => "AccessDenied".to_string(),
-            _ => io.to_string(),
-        },
+        Error::Io(io) => ghostty_os::zig_error::io_error_name(io)
+            .trim_start_matches("error.")
+            .to_owned(),
         e => e.to_string(),
     }
 }