Improve build system with debug symbols, clean modes, and parallel jobs

- Add build.md documentation for the WASM build system
- Default to debug builds with -gsource-map for WASM debugging
- Add --release flag to disable debug symbols
- Add --clean flag for full rebuild (deps + wxWidgets + KiCad)
- Add -j N flag for parallel compilation (default: 1 for sequential)
- Update Docker memory limit to 32GB
- Propagate DEBUG_BUILD and JOBS to all dependency scripts

Build modes:
- ./docker/build.sh --clean -j8  (full rebuild, parallel)
- ./docker/build.sh              (rebuild KiCad only)
- ./docker/build.sh --no-clean   (incremental)

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

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

scripts/deps/build-boost.sh

@@ -63,29 +63,38 @@ if [ ! -f "b2" ]; then
     ./bootstrap.sh --with-toolset=gcc
 fi
 
+# Determine boost variant based on DEBUG_BUILD
+if [ "${DEBUG_BUILD:-1}" = "1" ]; then
+    BOOST_VARIANT="debug"
+    BOOST_DEBUG_FLAGS="-g -O0"
+else
+    BOOST_VARIANT="release"
+    BOOST_DEBUG_FLAGS="-O2"
+fi
+
 # Create user-config.jam for Emscripten
-cat > user-config.jam << 'EOF'
+cat > user-config.jam << EOF
 using clang : emscripten
     : em++
-    : <cxxflags>"-pthread"
+    : <cxxflags>"${BOOST_DEBUG_FLAGS} -pthread"
       <linkflags>"-pthread"
     ;
 EOF
 
 # Build only the locale library (and its dependencies)
 # Most of Boost is header-only, we just need locale built
-log_info "Building Boost.Locale..."
-JOBS=${JOBS:-$(sysctl -n hw.ncpu 2>/dev/null || nproc 2>/dev/null || echo 4)}
+log_info "Building Boost.Locale (${BOOST_VARIANT})..."
+# JOBS is set in env.sh (default: 1 for sequential builds, use -j N to override)
 ./b2 -j${JOBS} \
     --user-config=user-config.jam \
     --prefix="${SYSROOT}" \
     --with-locale \
     toolset=clang-emscripten \
-    variant=release \
+    variant=${BOOST_VARIANT} \
     link=static \
     threading=multi \
     runtime-link=static \
-    cxxflags="-pthread" \
+    cxxflags="${BOOST_DEBUG_FLAGS} -pthread" \
     install
 
 create_stamp "${BOOST_STAMP}"