fix: +show-face u21/UTF-8 semantics, Zig output buffering, isAbsolute

Resolves RV14 (docs/rewrite/notes/review-rv14-codex.md, final-Z17.md):
- +show-face --cp parses as u21 (new U21 field type): out-of-range
  values give an invalid-value diagnostic and exit 1 instead of a panic;
  --string accepts raw bytes and fails with error.InvalidUtf8 after the
  codepoint lookup like Zig.
- CLI actions write through ZigWriter, a model of Zig 0.15's buffered
  File.writer (fixed buffer, drain on overflow, explicit flush), so
  stdout/stderr interleave in Zig's flush order (+show-face, help,
  version, validate-config, show-config, list-*, ssh-cache, crash-report,
  edit-config, new-window).
- Zig 0.15.2 std.fs.path.isAbsolute port (Windows: \theme and /theme are
  absolute) replaces Path::is_absolute at every Zig isAbsolute site.
- io_uring is available iff a 256-entry ring can be created, like
  libxev; the reactor falls back to IORING_OP_TIMEOUT without EXT_ARG.
- Windows basename (`λ:theme`) verified against 0.15.2 (test added).

Test Plan:
- cargo test --workspace --no-fail-fast
- diff +show-face outputs (pipe) against /usr/bin/ghostty

crates/ghostty-cli/src/validate_config.rs

@@ -29,9 +29,9 @@ pub fn run(args: &[impl AsRef<[u8]>]) -> Result<u8, Error> {
     let mut opts = Options::default();
     crate::parse_options(&mut opts, args)?;
 
-    let mut stdout = std::io::BufWriter::new(std::io::stdout().lock());
+    let mut stdout = crate::zig_writer::ZigWriter::new(crate::zig_writer::stdout_sink(), 1024);
     let result = run_inner(&opts, &mut stdout);
-    stdout.flush()?;
+    stdout.end()?;
     result
 }