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/src/index.ts

@@ -0,0 +1 @@
+export * from './zigjs';