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-bench/src/bin/ghostty-synthetic.rs

@@ -40,11 +40,14 @@ fn run() -> Result<(), String> {
         }
         "osc" => {
             let p_valid = args.take("p-valid", 0.5f64)?;
-            let style = match args.take_opt_string("style")?.as_deref() {
-                None | Some("streaming") => OscStyle::Streaming,
-                Some("parser") => OscStyle::Parser,
-                Some(s) => return Err(format!("invalid value for --style: {s}")),
-            };
+            let style = args.take_enum(
+                "style",
+                OscStyle::Streaming,
+                &[
+                    ("streaming", OscStyle::Streaming),
+                    ("parser", OscStyle::Parser),
+                ],
+            )?;
             args.finish()?;
             run_osc(&mut out, &mut rng, p_valid, style)
         }