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/tsconfig.json

@@ -0,0 +1,25 @@
+{
+  "compilerOptions": {
+    "alwaysStrict": true,
+    "sourceMap": false,
+    "strictNullChecks": true,
+    "strictFunctionTypes": true,
+    "strictPropertyInitialization": true,
+    "strictBindCallApply": true,
+    "module": "es2015",
+    "jsx": "react",
+    "target": "es5",
+    "moduleResolution": "node",
+    "experimentalDecorators": true,
+    "esModuleInterop": true,
+    "declaration": true,
+    "lib": ["dom", "es2017", "es5", "es6", "es7"],
+    "outDir": ".",
+    "strict": true,
+    "noImplicitAny": true,
+    "noImplicitThis": true,
+    "noImplicitReturns": true,
+    "skipLibCheck": true
+  },
+  "exclude": ["*.js", "tests", "examples", "src/*.test.tsx"]
+}