Multithreaded Callbacks

I'm working on the WebAssembly port of Box2D 3, which you can find here: [box2d3-wasm](https://github.com/Birch-san/box2d3-wasm).

We're aiming to make it feature-complete and highly flexible for web users. One challenge I'm currently facing involves supporting the [PreSolve callback](https://box2d.org/documentation/md_simulation.html#:~:text=race%20condition.-,Pre%2DSolve%20Callback,-This%20is%20called) in JavaScript.

Ideally, I want developers using the WASM build to be able to define a PreSolve callback directly in JavaScript, so they can make custom, real-time decisions on contact resolution.

Here's how I've set it up currently:
https://github.com/Birch-san/box2d3-wasm/blob/0a839337b041e1de851fc7be9e9a6aae6e4edc4e/box2d3-wasm/csrc/glue.cpp#L1562

This approach works perfectly in single-threaded builds and behaves exactly as intended. However, in multithreaded builds, I get the following runtime error:

```
Assertion failed: pthread_equal(thread, pthread_self()) && "val accessed from wrong thread"
```
Which makes sense, the callback is defined in JS, and Emscripten's val interface enforces that JavaScript calls only occur on the main thread for safety.

My question is: is there a known or recommended pattern for allowing JS-defined callbacks (like PreSolve) in a multithreaded Emscripten environment? Or is the best route simply to restrict this feature to single-threaded builds? ( I tinkered with proxying, but this resulted in a deadlock for me)

Any insights, suggestions, or pointers would be greatly appreciated!