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/cli_args.rs

@@ -605,6 +605,30 @@ impl_int_field!(
     isize => |_| CValue::Unsupported
 );
 
+/// Zig `u21` (a Unicode scalar range integer, `0..=0x1F_FFFF`).
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
+pub struct U21(pub u32);
+
+impl U21 {
+    pub const MAX: u32 = 0x1f_ffff;
+}
+
+impl ConfigField for U21 {
+    fn parse_bytes(&mut self, value: Option<&[u8]>) -> Result<(), ArgError> {
+        let v = value.ok_or(ArgError::ValueRequired)?;
+        let n = ghostty_os::zig_fmt::parse_int::<u32>(v, 0).map_err(|_| ArgError::InvalidValue)?;
+        if n > Self::MAX {
+            return Err(ArgError::InvalidValue);
+        }
+        *self = U21(n);
+        Ok(())
+    }
+
+    fn format_entry(&self, f: &mut EntryFormatter<'_>) -> fmt::Result {
+        writeln!(f.writer, "{} = {}", f.name, self.0)
+    }
+}
+
 macro_rules! impl_float_field {
     ($($t:ty => $cv:expr),*) => {$(
         impl ConfigField for $t {
@@ -1021,6 +1045,29 @@ mod tests {
         }
     }
 
+    config_struct! {
+        pub struct Cp {
+            "cp" => cp: Option<U21> = None,
+        }
+    }
+
+    #[test]
+    fn u21_field_matches_zig_range() {
+        let mut c = Cp::raw_default();
+        c.parse_field("cp", Some("0x1FFFFF")).unwrap();
+        assert_eq!(c.cp, Some(U21(0x1f_ffff)));
+        c.parse_field("cp", Some("0b1000001")).unwrap();
+        assert_eq!(c.cp, Some(U21(65)));
+        for bad in ["0x200000", "4294967295", "99999999999999999999", "-1", "x"] {
+            assert_eq!(
+                c.parse_field("cp", Some(bad)),
+                Err(ArgError::InvalidValue),
+                "{bad}"
+            );
+        }
+        assert_eq!(c.parse_field("cp", None), Err(ArgError::ValueRequired));
+    }
+
     config_enum! {
         pub enum OneTwo { One => "one", Two => "two" }
     }