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

docs/rewrite/notes/final-Z14.md

@@ -0,0 +1,5 @@
+# Z14 (RV11 gaps)
+
+1. ghostty-bench: `Args::take_enum` validates every occurrence of an enum option in order (empty value resets to default), used by codepoint-width, grapheme-break, is-symbol, screen-clone, terminal-stream `--handler` and synthetic `+osc --style`. Tests in args.rs / benches.rs.
+2. Theme open (named and absolute) uses `to_syscall_path` so an interior NUL truncates the path like Zig's C string. Test `theme_absolute_interior_nul_opens_prefix`; checked against /usr/bin/ghostty (no diagnostics, loads).
+3. New `ghostty_os::zig_error::{errno_name, io_error_name}` (Zig std posix errno mapping). Used by theme.rs, path.rs, config.rs (config-file open, `config::Error::Io` Display), file_load.rs logs, edit_config.rs, list_themes preview. Verified identical to oracle for FileNotFound, NotDir, AccessDenied, IsDir ("it is a directory" path), NameTooLong, SymLinkLoop.