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-termio/src/shell_integration.rs

@@ -532,6 +532,17 @@ mod tests {
         assert_eq!(detect_shell(&sh("")), None);
     }
 
+    #[test]
+    fn detect_shell_splits_on_all_zig_separators() {
+        for sep in ["\n", "\r", "\t", " "] {
+            let v: Vec<_> = sh(&format!("bash{sep}-i")).arg_iterator().collect();
+            assert_eq!(v, ["bash", "-i"]);
+            assert_eq!(detect_shell(&sh(&format!("zsh{sep}-l"))), Some(Shell::Zsh));
+        }
+        // The first argument must end at the LF, not include `-i`.
+        assert_eq!(detect_shell(&sh("/bin/fish\n-l")), Some(Shell::Fish));
+    }
+
     #[test]
     fn arg_iterator_shell_and_direct() {
         let v: Vec<_> = sh("echo hello world").arg_iterator().collect();