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-config/src/theme.rs

@@ -229,7 +229,7 @@ pub fn open(
     let theme_text = theme.as_encoded();
 
     // Absolute themes are loaded a different path.
-    if theme_path.is_absolute() {
+    if theme.is_zig_absolute() {
         let file = open_absolute(theme, diags)?;
         if !check_regular(&file, theme_text, diags) {
             return None;
@@ -446,6 +446,21 @@ mod tests {
         assert_eq!(zig_basename("./"), ".");
     }
 
+    #[test]
+    fn windows_basename_matches_zig_0_15_2() {
+        // The drive colon is recognized only at byte index 1.
+        assert_eq!(basename_windows("\u{3bb}:theme"), "\u{3bb}:theme");
+        assert_eq!(
+            basename_windows("light:\u{3bb}:theme"),
+            "light:\u{3bb}:theme"
+        );
+        assert_eq!(basename_windows("1:theme"), "theme");
+        assert_eq!(basename_windows("c:"), "");
+        assert_eq!(basename_windows("a\\b/c\\"), "c");
+        assert_eq!(basename_windows("\\"), "");
+        assert_eq!(basename_windows("theme"), "theme");
+    }
+
     #[test]
     fn dot_theme_names_are_directories() {
         let dir = std::env::temp_dir().join(format!("ghostty-theme-dot-{}", std::process::id()));