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-os/src/zig_error.rs

@@ -0,0 +1,63 @@
+//! `std::io::Error` -> Zig error name (`{}` of an error value, e.g.
+//! `error.FileNotFound`), following Zig std's posix errno mapping for the
+//! calls Ghostty makes (`openat`, `fstat`, `realpath`, `execve`, ...).
+
+/// Zig error name (without the `error.` prefix) for a Linux errno.
+#[must_use]
+pub fn errno_name(errno: i32) -> &'static str {
+    match errno {
+        1 => "PermissionDenied",        // EPERM
+        2 => "FileNotFound",            // ENOENT
+        3 => "ProcessNotFound",         // ESRCH
+        6 | 19 => "NoDevice",           // ENXIO, ENODEV
+        11 => "WouldBlock",             // EAGAIN
+        12 => "SystemResources",        // ENOMEM
+        13 => "AccessDenied",           // EACCES
+        16 => "DeviceBusy",             // EBUSY
+        17 => "PathAlreadyExists",      // EEXIST
+        20 => "NotDir",                 // ENOTDIR
+        21 => "IsDir",                  // EISDIR
+        23 => "SystemFdQuotaExceeded",  // ENFILE
+        24 => "ProcessFdQuotaExceeded", // EMFILE
+        26 => "FileBusy",               // ETXTBSY
+        27 | 75 => "FileTooBig",        // EFBIG, EOVERFLOW
+        28 => "NoSpaceLeft",            // ENOSPC
+        30 => "ReadOnlyFileSystem",     // EROFS
+        36 => "NameTooLong",            // ENAMETOOLONG
+        40 => "SymLinkLoop",            // ELOOP
+        84 => "InvalidWtf8",            // EILSEQ
+        95 => "FileLocksNotSupported",  // EOPNOTSUPP
+        _ => "Unexpected",
+    }
+}
+
+/// `error.Name` text for an I/O error.
+#[must_use]
+pub fn io_error_name(err: &std::io::Error) -> String {
+    if let Some(errno) = err.raw_os_error() {
+        return format!("error.{}", errno_name(errno));
+    }
+    match err.kind() {
+        std::io::ErrorKind::NotFound => "error.FileNotFound".into(),
+        std::io::ErrorKind::PermissionDenied => "error.AccessDenied".into(),
+        std::io::ErrorKind::InvalidInput => "error.BadPathName".into(),
+        _ => "error.Unexpected".into(),
+    }
+}
+
+#[cfg(all(test, unix))]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn names() {
+        let n = |e| io_error_name(&std::io::Error::from_raw_os_error(e));
+        assert_eq!(n(2), "error.FileNotFound");
+        assert_eq!(n(20), "error.NotDir");
+        assert_eq!(n(13), "error.AccessDenied");
+        assert_eq!(n(21), "error.IsDir");
+        assert_eq!(n(36), "error.NameTooLong");
+        assert_eq!(n(40), "error.SymLinkLoop");
+        assert_eq!(n(9999), "error.Unexpected");
+    }
+}