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-os/src/osbytes.rs

@@ -431,6 +431,17 @@ impl EncodedOs {
         }
     }
 
+    /// [`EncodedOs::to_path`] as a syscall sees it: Zig hands paths to the
+    /// OS as NUL-terminated C strings, so an interior NUL ends the path
+    /// (Rust's `std::fs` would refuse the whole path instead).
+    pub fn to_syscall_path(&self) -> std::io::Result<std::path::PathBuf> {
+        let bytes = self.to_bytes();
+        match bytes.iter().position(|&b| b == 0) {
+            Some(end) => EncodedOs::from_bytes(&bytes[..end]).to_path(),
+            None => self.to_path(),
+        }
+    }
+
     /// Whether the decoded value starts with the given plain prefix.
     #[must_use]
     pub fn starts_with_plain(&self, prefix: &str) -> bool {
@@ -526,6 +537,22 @@ pub fn config_str_to_path(s: &str) -> std::io::Result<std::path::PathBuf> {
 mod tests {
     use super::*;
 
+    #[cfg(unix)]
+    #[test]
+    fn syscall_path_ends_at_the_first_nul() {
+        let e = EncodedOs::from_bytes(b"/a/b\0c");
+        assert_eq!(
+            e.to_syscall_path().unwrap(),
+            std::path::PathBuf::from("/a/b")
+        );
+        let e = EncodedOs::from_bytes(b"/a/\xffb");
+        assert_eq!(e.to_syscall_path().unwrap(), e.to_path().unwrap());
+        assert_eq!(
+            EncodedOs::from_bytes(b"\0x").to_syscall_path().unwrap(),
+            std::path::PathBuf::new()
+        );
+    }
+
     #[test]
     fn windows_paths_fail_with_bad_path_name_instead_of_replacement() {
         let error = windows_path_wide(b"C:\\bad\xff").unwrap_err();