feat(build): complete WASM package build scripts

- yoga-layout: Add WASM linking with Emscripten bindings
- onnx-runtime: Add Homebrew Emscripten detection fallback
- minilm-builder: Implement full ML pipeline with Python scripts
- toolchain: Add Python ML dependencies (transformers, optimum, onnx)
- gitignore: Exclude build artifacts, checkpoints, and downloaded sources

Enables building optimized WASM modules for layout engine, ONNX Runtime,
and MiniLM embedding models. Supports both EMSDK and Homebrew installations
of Emscripten for better developer experience.

Author: jdalton

.gitignore

@@ -24,6 +24,7 @@ Thumbs.db
 **/dist
 **/build
 **/node_modules
+.build-checkpoints
 **/pkg
 **/target
 *.d.ts
@@ -42,3 +43,8 @@ Thumbs.db
 
 # Ignore platform binary build artifacts
 packages/socketbin-*/bin/
+
+# Ignore downloaded build sources
+packages/*/.yoga-source
+packages/*/.onnx-source
+packages/*/.minilm-source

packages/minilm-builder/.gitignore

@@ -6,3 +6,6 @@ dist/
 
 # Node modules.
 node_modules/
+
+# Auto-generated Python scripts.
+python/