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-lib/src/action_c.rs

@@ -34,16 +34,15 @@ pub struct Keep {
 }
 
 impl Keep {
-    /// A NUL terminated copy of `s` (interior NULs are dropped).
+    /// A NUL terminated copy of `s` (it ends at the first interior NUL).
     fn cstr(&mut self, s: &str) -> *const c_char {
         self.cstr_bytes(s.as_bytes())
     }
 
     /// [`Keep::cstr`] for raw bytes.
     fn cstr_bytes(&mut self, s: &[u8]) -> *const c_char {
-        let mut s = s.to_vec();
-        s.retain(|&b| b != 0);
-        let c = CString::new(s).unwrap_or_default();
+        let end = s.iter().position(|&b| b == 0).unwrap_or(s.len());
+        let c = CString::new(&s[..end]).unwrap_or_default();
         let ptr = c.as_ptr();
         // The heap buffer of a CString doesn't move when the Vec grows.
         self.strings.push(c);
@@ -241,6 +240,26 @@ mod tests {
 
     use super::*;
 
+    #[test]
+    fn c_title_actions_end_at_the_first_interior_nul() {
+        use ghostty_os::osbytes::EncodedOs;
+        let mut keep = Keep::default();
+        for tab in [false, true] {
+            let title = SetTitle {
+                title: EncodedOs::from_bytes(b"a\0b"),
+            };
+            let action = if tab {
+                Action::SetTabTitle(title)
+            } else {
+                Action::SetTitle(title)
+            };
+            let c = action_to_c(&action, &mut keep);
+            // SAFETY: both title union members have identical layout, pointer owned by keep.
+            let title = unsafe { std::ffi::CStr::from_ptr(c.action.set_title.title) };
+            assert_eq!(title.to_bytes(), b"a");
+        }
+    }
+
     #[test]
     fn c_title_actions_and_key_table_names_forward_original_bytes() {
         use ghostty_os::osbytes::EncodedOs;