fix(wasm): Fix asyncify rewind with asyncify-aware dynCall shims

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>

scripts/common/functions.sh

@@ -53,7 +53,7 @@ setup_error_trap() {
 # Verify Emscripten is available
 verify_emscripten() {
     if ! command -v emcc &> /dev/null; then
-        log_error "Emscripten not found. Please run: source /path/to/emsdk/emsdk_env.sh"
+        log_error "Emscripten not found. Run: ./scripts/setup-emsdk.sh"
         exit 1
     fi
     log_info "Using Emscripten: $(emcc --version | head -1)"