feat(wasm): use zig-js's exact JS interop ABI

Ghostty's wasm code talks to JavaScript through the zig-js library
(mitchellh/zig-js, pinned in build.zig.zon). The first port invented its
own import ABI because the dependency wasn't available; now:
- ghostty-os/src/wasm/js.rs is a Rust port of zig-js's Zig side (Ref,
  Value, Object get/set/call/new, global, runtime, string) using its
  "zig-js" import module, function names and NaN-boxed value refs.
- web_canvas face and the atlas debug canvas make the same JS calls as
  Ghostty's Zig code.
- zig-js's unmodified JS host (+ LICENSE) is vendored under
  crates/ghostty-wasm/js/zig-js and drives the node test.

Test Plan:
- cargo test -p ghostty-wasm (node >= 24; asserts zig-js imports and no
  leaked host values)

crates/ghostty-wasm/src/lib.rs

@@ -11,7 +11,7 @@
 //! - module `config`, `config/Wasm.zig` (`config_*`)
 //! - module `app`, `App.zig` `Wasm` (commented out upstream, nothing exported)
 //!
-//! The host provides the `ghostty_js` imports (see
+//! The host provides the `zig-js` imports (see
 //! `ghostty_os::wasm::js`) and `env.log`. Strings are passed as
 //! pointer+length pairs into wasm memory allocated by the host with
 //! [`wasm_alloc`] / released with [`wasm_free`].