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/path.rs

@@ -164,7 +164,11 @@ impl Path {
             }
             Err(err) => {
                 diags.append(Diagnostic {
-                    message: format!("error resolving file path {}: {err}", path.as_encoded()),
+                    message: format!(
+                        "error resolving file path {}: {}",
+                        path.as_encoded(),
+                        ghostty_os::zig_error::io_error_name(&err)
+                    ),
                     ..Diagnostic::default()
                 });