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-config/src/file_load.rs

@@ -170,15 +170,20 @@ pub fn open(path: &EncodedOs) -> Result<File, OpenFileError> {
             OpenFileError::FileNotFound
         } else {
             log::warn!(
-                "unexpected file open error path={} err={err}",
-                real.display()
+                "unexpected file open error path={} err={}",
+                real.display(),
+                ghostty_os::zig_error::io_error_name(&err)
             );
             OpenFileError::FileOpenFailed
         }
     })?;
 
     let meta = file.metadata().map_err(|err| {
-        log::warn!("error getting file stat path={} err={err}", real.display());
+        log::warn!(
+            "error getting file stat path={} err={}",
+            real.display(),
+            ghostty_os::zig_error::io_error_name(&err)
+        );
         OpenFileError::FileOpenFailed
     })?;
     if !meta.is_file() {