Add two-phase build with host asyncify transformation

- Move asyncify from Docker to host to avoid memory issues
- Auto-download Binaryen v121 (v125 has regression bug)
- Use -O1 for debug builds (V8 local count limit)
- Remove asyncify flags from linker (handled by wasm-opt)
- Document two-phase build in build.md

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docker/build.sh

@@ -1,13 +1,25 @@
 #!/bin/bash
-# Build KiCad WASM inside Docker container
+# Build KiCad WASM inside Docker container, then apply asyncify on host
+#
+# The build is split into two phases:
+# 1. Docker: Compile KiCad to WASM (without asyncify)
+# 2. Host: Apply asyncify transformation (uses Binaryen v121)
+#
+# Binaryen is downloaded automatically - no prerequisites needed.
+
 set -e
 
 cd "$(dirname "$0")/.."
 
+# Get wasm-opt (downloads Binaryen v121 if not cached)
+echo "Checking wasm-opt..."
+WASM_OPT=$(./scripts/common/get-wasm-opt.sh)
+echo "Using: ${WASM_OPT}"
+
 # Start container if not running
 docker compose -f docker/docker-compose.yml up -d
 
-# Run build command
+# Run build command (without asyncify - handled on host due to memory requirements)
 docker compose -f docker/docker-compose.yml exec kicad-wasm-builder \
     /workspace/scripts/kicad/build-pcbnew.sh "$@"
 
@@ -16,4 +28,23 @@ echo "Copying build output to ./output/..."
 docker compose -f docker/docker-compose.yml exec kicad-wasm-builder \
     bash -c "mkdir -p /workspace/output && cp /workspace/build-wasm/kicad-pcbnew/pcbnew/pcbnew.{js,wasm,wasm.map,worker.js} /workspace/output/ 2>/dev/null || cp /workspace/build-wasm/kicad-pcbnew/pcbnew/pcbnew.{js,wasm} /workspace/output/"
 
+# Apply asyncify transformation on host
+# This is done on the host because wasm-opt --asyncify needs 50GB+ RAM for KiCad,
+# which exceeds typical Docker memory limits
+echo ""
+echo "Applying asyncify transformation on host..."
+echo "This may take several minutes and use significant RAM..."
+
+# Asyncify import patterns (functions that trigger async suspension)
+# - env.invoke_* : Exception handling trampolines
+# - env.__asyncjs__* : EM_ASYNC_JS functions
+ASYNCIFY_IMPORTS="env.invoke_*,env.__asyncjs__*"
+
+"${WASM_OPT}" --asyncify \
+    --pass-arg=asyncify-imports@${ASYNCIFY_IMPORTS} \
+    --pass-arg=asyncify-propagate-addlist \
+    output/pcbnew.wasm -o output/pcbnew.wasm
+
+echo ""
 echo "Build complete. Output files in ./output/"
+ls -lh output/