Emscripten 4.x removed dynCall_* WASM exports, breaking asyncify
rewind through indirect calls (modal dialogs, event handlers).
Generate JS shims that track Asyncify.exportCallStack and register
in wasmExports so doRewind can find them.
Also fixes empty callback functions ((() => {})) generated by
Emscripten 4.x + pthreads for HTML5 events, pthread entry,
sighandler, async timer, and main loop callbacks.
Build pipeline improvements:
- Stub wasm-opt/finalize in Docker (RAM limits), run on host
- Add setup-emsdk.sh for reproducible Emscripten setup
- Simplify env.sh and version management
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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