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/args.rs

@@ -122,6 +122,27 @@ impl Args {
         })
     }
 
+    /// Take an enum value (`parseIntoField` on an enum): every occurrence is
+    /// validated against `variants` in order (a bad value is an error even if
+    /// a later one is fine); an empty value resets to `default`.
+    pub fn take_enum<T: Copy>(
+        &mut self,
+        key: &str,
+        default: T,
+        variants: &[(&str, T)],
+    ) -> Result<T, String> {
+        self.apply(key, default, |v| {
+            let v = v.ok_or_else(|| format!("option --{key} requires a value"))?;
+            let bad = || format!("invalid value for --{key}: {}", v.to_string_lossy());
+            let v = v.to_str().ok_or_else(bad)?;
+            variants
+                .iter()
+                .find(|(n, _)| *n == v)
+                .map(|(_, t)| *t)
+                .ok_or_else(bad)
+        })
+    }
+
     /// Take a file path value (any bytes are allowed).
     pub fn take_opt_path(&mut self, key: &str) -> Result<Option<PathBuf>, String> {
         self.apply(key, None, |v| {
@@ -197,6 +218,21 @@ mod tests {
         assert!(a.take("n", 0u16).is_err());
     }
 
+    #[test]
+    fn take_enum_validates_every_occurrence() {
+        let t = |v: &[&str]| {
+            Args::parse(v.iter().map(OsString::from))
+                .unwrap()
+                .take_enum("mode", 0u8, &[("noop", 0), ("table", 1)])
+        };
+        assert!(t(&["--mode=bad", "--mode=noop"]).is_err());
+        assert!(t(&["--mode=noop", "--mode=bad"]).is_err());
+        assert!(t(&["--mode=bad", "--mode="]).is_err());
+        assert_eq!(t(&["--mode=table", "--mode="]).unwrap(), 0);
+        assert_eq!(t(&["--mode=noop", "--mode=table"]).unwrap(), 1);
+        assert_eq!(t(&[]).unwrap(), 0);
+    }
+
     #[test]
     fn duplicates_apply_in_order_and_first_error_wins() {
         let a = |v: &[&str]| Args::parse(v.iter().map(OsString::from)).unwrap();