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/setup-emsdk.sh

@@ -0,0 +1,60 @@
+#!/bin/bash
+# Install and activate the Emscripten SDK (emsdk) locally
+#
+# This script is idempotent - it skips installation if the correct version
+# is already installed. The emsdk is installed into tools/emsdk/ which is
+# gitignored (~1.5GB).
+#
+# Usage:
+#   ./scripts/setup-emsdk.sh          # Install/activate pinned version
+#   ./scripts/setup-emsdk.sh --force  # Force reinstall
+
+set -e
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+PROJECT_ROOT="$(dirname "$SCRIPT_DIR")"
+
+# Source version constants
+source "$SCRIPT_DIR/common/versions.sh"
+
+EMSDK_DIR="$PROJECT_ROOT/tools/emsdk"
+VERSION="$EMSCRIPTEN_VERSION"
+
+echo "=== Emscripten SDK Setup ==="
+echo "  Version: $VERSION"
+echo "  Location: $EMSDK_DIR"
+
+# Check if already installed with correct version (unless --force)
+if [ "$1" != "--force" ] && [ -f "$EMSDK_DIR/emsdk" ]; then
+    if "$EMSDK_DIR/emsdk" list 2>/dev/null | grep -q "^[[:space:]]*${VERSION}[[:space:]]*INSTALLED"; then
+        echo "Emscripten $VERSION is already installed and active."
+        echo "Use --force to reinstall."
+        exit 0
+    fi
+fi
+
+# Clone emsdk if not present
+if [ ! -d "$EMSDK_DIR" ]; then
+    echo ""
+    echo "Cloning emsdk..."
+    mkdir -p "$PROJECT_ROOT/tools"
+    git clone https://github.com/emscripten-core/emsdk.git "$EMSDK_DIR"
+else
+    echo ""
+    echo "Updating emsdk..."
+    (cd "$EMSDK_DIR" && git pull)
+fi
+
+# Install and activate the pinned version
+echo ""
+echo "Installing Emscripten $VERSION..."
+"$EMSDK_DIR/emsdk" install "$VERSION"
+
+echo ""
+echo "Activating Emscripten $VERSION..."
+"$EMSDK_DIR/emsdk" activate "$VERSION"
+
+echo ""
+echo "=== Setup complete ==="
+echo "Emscripten $VERSION is ready."
+echo "Source env.sh to use it: source scripts/common/env.sh"