fix(config): match Zig's numeric grammar, NUL handling and diagnostics

Resolves the 6 differential gap groups in docs/rewrite/notes/review-
rv9-codex.md (details in final-Z11.md):
- ghostty_os::zig_fmt ports std.fmt.parseInt/parseUnsigned/parseFloat
  (underscores, 0x/0o/0b prefixes, hex floats, inf/nan) with the Zig std
  tests and replaces every int/float parse in config, keybind
  parameters, ssh-cache and desktop detection.
- Interior NUL no longer panics in font names and ends C strings like
  Zig's backing bytes (fontconfig, C config getters, title actions,
  clipboard callback, relative paths, config-file, shaders, background
  image, GTK CSS).
- GTK new-window arguments are trimmed of ASCII whitespace only.
- Command tokenization uses a port of std.process.ArgIteratorGeneral
  (incl. CR/LF separators) with its std tests.
- UTF-8 validation happens where Zig's field parsers do it, so error
  kinds match (metric modifiers, quick-terminal size, link, ...).

A scripted differential run of 202 fields x ~220 hostile/valid values
against /usr/bin/ghostty (+validate-config and +show-config: exit code,
stdout, stderr) now has zero mismatches besides build-specific defaults
(theme resources path, auto-update-channel, gtk-opengl-debug); it also
surfaced and fixed background-blur failure state, NaN formatting and
NaN/-0 finalize clamps. Regression tests: ghostty-config/tests/
zig_parity.rs.

Test Plan:
- cargo test --workspace --no-fail-fast (3456 passed, exit 0)
- just check-macos (both), just check-windows, just check-wasm

crates/ghostty-config/src/path.rs

@@ -154,7 +154,7 @@ impl Path {
         // Make sure the base directory can be opened.
         std::fs::read_dir(&base_path)?;
 
-        let full = base_path.join(path.to_path()?);
+        let full = base_path.join(path.to_syscall_path()?);
         let abs = match std::fs::canonicalize(&full) {
             Ok(p) => EncodedOs::from_path(&p),
             Err(err) if err.kind() == std::io::ErrorKind::NotFound => {
@@ -392,6 +392,31 @@ mod tests {
         assert!(diags.is_empty());
     }
 
+    #[cfg(unix)]
+    #[test]
+    fn expand_interior_nul_resolves_the_c_string_prefix_like_zig() {
+        let tmp = ghostty_os::TempDir::init().unwrap();
+        let base = std::fs::canonicalize(tmp.path()).unwrap();
+        std::fs::write(base.join("exists"), b"x").unwrap();
+
+        // The syscall sees only the prefix, which exists: the real path of
+        // the prefix replaces the value.
+        let mut p = Path::Required(EncodedOs::from_bytes(b"exists\0tail"));
+        let mut diags = DiagnosticList::default();
+        p.expand(&EncodedOs::from_path(&base), &mut diags).unwrap();
+        assert_eq!(diags.items().len(), 0);
+        assert_eq!(p.path().to_path().unwrap(), base.join("exists"));
+
+        // The prefix is missing: the whole value is resolved lexically.
+        let mut p = Path::Required(EncodedOs::from_bytes(b"missing\0tail"));
+        p.expand(&EncodedOs::from_path(&base), &mut diags).unwrap();
+        assert_eq!(diags.items().len(), 0);
+        assert_eq!(
+            p.path().to_bytes(),
+            [base.as_os_str().as_encoded_bytes(), b"/missing\0tail"].concat()
+        );
+    }
+
     #[cfg(unix)]
     #[test]
     fn expand_symlink_to_non_utf8_target_round_trips() {