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-harfbuzz.sh

@@ -57,9 +57,11 @@ cd "${HARFBUZZ_BUILD}"
 
 # HarfBuzz uses meson, but also has CMake support
 emcmake cmake "${HARFBUZZ_DIR}" \
-    -DCMAKE_BUILD_TYPE=Release \
+    -DCMAKE_BUILD_TYPE=${BUILD_TYPE:-Debug} \
     -DCMAKE_INSTALL_PREFIX="${SYSROOT}" \
     -DCMAKE_POLICY_VERSION_MINIMUM=3.5 \
+    -DCMAKE_C_FLAGS="${DEBUG_CFLAGS:--g -O0}" \
+    -DCMAKE_CXX_FLAGS="${DEBUG_CFLAGS:--g -O0}" \
     -DHB_HAVE_FREETYPE=ON \
     -DHB_HAVE_GLIB=OFF \
     -DHB_HAVE_ICU=OFF \