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-renderer/src/generic.rs

@@ -244,7 +244,7 @@ impl DerivedConfig {
             .value
             .iter()
             .filter_map(|p| {
-                let path = match p.path().to_path() {
+                let path = match p.path().to_syscall_path() {
                     Ok(path) => path,
                     Err(error) => {
                         log::warn!("invalid shader path: {error}");
@@ -869,7 +869,7 @@ impl<G: GraphicsApi> DrawState<G> {
         if let Some(p) = &self.config.bg_image
             && let Some(image) = p
                 .path()
-                .to_path()
+                .to_syscall_path()
                 .map_err(|error| log::warn!("invalid background image path: {error}"))
                 .ok()
                 .and_then(|path| load_background_image(&path))