fix: use Zig's numeric parsing in protocol and escape-sequence parsers

Rust's str::parse accepts and rejects different inputs than Zig's
std.fmt.parseInt/parseFloat (underscores, base prefixes, signs). The
config layer already used the Zig ports; now the protocol parsers do
too (call site -> Zig reference -> status in final-Z12.md):
- zig_fmt moves into the leaf crate ghostty-zigfmt (ghostty-vt cannot
  depend on ghostty-os); ghostty-os re-exports it unchanged.
- ghostty-vt: kitty graphics keys, kitty text sizing, OSC 4/5/10-19
  color indices, hex color components and intensities, OSC 9 sleep and
  progress, OSC 133 exit codes, OSC 3008 fields, kitty color palette.
- ghostty-terminal: tmux layout, format variables and pane tabs.
- ghostty-os: URI ports and %XX decoding; ghostty-bench CLI arguments.
Expected values were cross-checked against the real Zig toolchain.

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

Cargo.lock

@@ -1618,6 +1618,7 @@ dependencies = [
  "crossbeam-channel",
  "gettext-rs",
  "ghostty-vt",
+ "ghostty-zigfmt",
  "libc",
  "log",
  "nix 0.31.3",
@@ -1698,6 +1699,7 @@ dependencies = [
  "ghostty-os",
  "ghostty-unicode",
  "ghostty-vt",
+ "ghostty-zigfmt",
  "log",
  "onig",
  "png",
@@ -1740,6 +1742,7 @@ name = "ghostty-vt"
 version = "0.1.0"
 dependencies = [
  "base64 0.23.1",
+ "ghostty-zigfmt",
  "log",
  "memchr",
  "thiserror 2.0.21",
@@ -1766,6 +1769,10 @@ dependencies = [
  "temp-env",
 ]
 
+[[package]]
+name = "ghostty-zigfmt"
+version = "0.1.0"
+
 [[package]]
 name = "gio"
 version = "0.22.10"