Fix embind exports with MODULARIZE=instance and AUTO_INIT

[guybedford]

This fixes embind ES module exports being undefined when using -sMODULARIZE=instance -sAUTO_INIT -sEMBIND_AOT, resolves #27411.

With AUTO_INIT the module self-initializes via top-level await init() in the postamble, but the embind AOT export snippet was appended after the postamble. Its addOnPostCtor(assignEmbindExports) therefore registered only after onPostCtors had already fired during init, so the export assignments never ran.

Following review feedback, rather than supporting late registration, the snippet is now emitted via a <<< EMBIND_AOT_EXPORTS >>> placeholder at the start of the MODULARIZE=instance postamble (same mechanism as EMBIND_AOT_INVOKERS), so registration always precedes initialization. addOnPostCtor additionally asserts under ASSERTIONS that it is not called after ctors have run, rather than silently never firing.

Adds other.test_modularize_instance_auto_init_embind reusing modularize_instance_embind.cpp, verified to fail before the fix and pass after; existing modularize=instance, ESM integration, dylink and codesize tests still pass (also manually verified WASM_ESM_INTEGRATION + AUTO_INIT + EMBIND_AOT under node 26).

Made with AI assistance under my review

[sbc100]

I would hope that we don't need to do this (instead maybe we could make it an assertion if this function is called too late), and we can instread find a way to do the registration before startup. Otherwise we might be tempted to support other event handler beeing registered too late too.

@brendandahl WDYT?

[guybedford]

I've updated the approach so postCtor events can only be registered before startup.