[Feature] FFI closure direction 2: marshal JS functions into wasm (callback args / nested results)

[katsujukou]

Summary

A JS function passed into wasm — as a callback argument, or nested inside a foreign's
result — cannot currently be marshalled into a callable wasm closure. Only direction 1 (a
wasm closure handed out to JS) is implemented.

Status / evidence

• The import-direction marshaller throws explicitly: bin/src/Main.purs:465 —
  "marshalling a JS function into wasm is not yet supported (ADR 0014, closure direction 2)".
• ADR 0014 defers this: it needs a JS-side function registry + a host-import trampoline so
  wasm can hold and re-enter a JS callable.

Direction

Per ADR 0014: register JS callables and expose a host-import trampoline; have eqrefFromJs
wrap a JS function as a wasm closure that calls back through it.