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/config.rs

@@ -3935,7 +3935,8 @@ impl Config {
     ///
     /// `path` must be resolved and absolute.
     pub fn load_file(&mut self, path: &EncodedOs) -> Result<(), Error> {
-        assert!(path.to_path()?.is_absolute());
+        path.to_path()?;
+        assert!(path.is_zig_absolute());
         let file = match crate::file_load::open(path) {
             Ok(f) => f,
             Err(crate::file_load::OpenFileError::NotAFile) => {
@@ -4260,7 +4261,8 @@ impl Config {
 
             // All paths should already be absolute at this point because
             // they're fixed up after each load.
-            assert!(path.to_path()?.is_absolute());
+            path.to_path()?;
+            assert!(path.is_zig_absolute());
 
             // We must only load a unique file once
             if !loaded.insert(path.clone()) {