Fix pthread deadlock by using dynamic PTHREAD_POOL_SIZE

The KiCad WASM build was freezing during initialization due to a pthread
deadlock. When hardware_concurrency() > PTHREAD_POOL_SIZE, new Web Worker
creation requires the event loop, but the main thread busy-waits blocking it.

Changes:
- Use navigator.hardwareConcurrency for PTHREAD_POOL_SIZE in KiCad build
- Add threadpool test to reproduce and verify the fix
- Add single-target build support to build-wasm-test.sh
- Default docker/build.sh to -j 10 for faster builds

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

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

docker/build.sh

@@ -11,12 +11,18 @@ set -e
 
 cd "$(dirname "$0")/.."
 
+# Add -j 10 by default if no -j flag is given
+ARGS=("$@")
+if [[ ! " ${ARGS[*]} " =~ " -j " ]]; then
+    ARGS+=("-j" "10")
+fi
+
 # Start container if not running
 docker compose -f docker/docker-compose.yml up -d
 
 # 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 "$@"
+    /workspace/scripts/kicad/build-pcbnew.sh "${ARGS[@]}"
 
 # Copy output to host-accessible directory
 echo "Copying build output to ./output/..."