Wasm: dispatch lifecycle events when async instantiation completes

[JasonGross]

Summary

• Fixes [BUG] wasm is not ready when DOMContentLoaded fires and does not trigger an event when it is loaded #11: surrounding JavaScript has no way to know when the asynchronous Wasm instantiation has finished, so user code that runs from DOMContentLoaded can race against the OCaml exports being defined.
• The generated launcher invokes the runtime as init_fun(primitives)(runtime_arguments) and discards the returned Promise, so callers can't simply await it. Instead, the runtime now dispatches a wasmoocaml:loaded CustomEvent on globalThis after _initialize() resolves, and a wasmoocaml:error event (with the thrown error in detail.error) if anything in the instantiation pipeline rejects.
• Both events carry detail.src (the assets directory name passed in args.src) so listeners can disambiguate when multiple Wasm modules are loaded on the same page.
• Listener example:

  addEventListener("wasmoocaml:loaded", (e) => {
    // OCaml exports are now available; e.detail.src identifies the module.
    synthesize(...);
  });

Test plan

• node --check runtime/wasm/runtime.js passes (syntax check).
• npx -p @biomejs/biome@1.9.1 biome check runtime/wasm/runtime.js passes.
• Manual verification with the fiat-html reproducer from [BUG] wasm is not ready when DOMContentLoaded fires and does not trigger an event when it is loaded #11: replace the setTimeout(..., 1) with a wasmoocaml:loaded listener and confirm synthesize is defined when the handler fires.

🤖 Generated with Claude Code

[hhugo]

You probably want to open a PR against https://github.com/ocsigen/js_of_ocaml

[JasonGross]

Thank you, doing that now

[JasonGross]

Superseded by ocsigen/js_of_ocaml#2232, which ports this change to the merged js_of_ocaml repository. Closing here.