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/js/zig-js/README.md

@@ -0,0 +1,21 @@
+# zig-js Glue
+
+This is the JS library that glues with the Zig library so that Zig
+can communicate with the host JS environment when compiled to WebAssembly.
+Learn more about the zig-js library at the
+[zig-js repository](https://github.com/mitchellh/zig-js).
+
+## Installation
+
+There are two methods for using the JS side of zig-js:
+
+1. This library is a standard npm package, so you can install and use it
+   as you normally would any other package. Add it to your package.json and
+   integrate it with the bundler you're already using.
+
+2. You can run `npm run build` in this directory and the library will be
+   bundled into a single JS file in `dist/`. You can then copy the single JS
+   file into your web project. We bundle both a normal CJS
+   module at `dist/index.js` as well as an ES module at `dist/module.js`.
+   TypeScript type definitions are available in `dist/types.d.ts`.
+