ge-hch.5.15.16: add embedding service

[SorraTheOrc]

Summary

• add browser-first embedding service that lazily loads Xenova/all-MiniLM-L6-v2 via transformers.js in a Web Worker
• expose embed(text) with graceful fallback and similarity(vecA, vecB) cosine helper
• keep failures non-fatal (null) for unsupported environments/test contexts

Testing

• npx jest tests/unit/director.test.js --runInBand

Closes bd#ge-hch.5.15.16