Core optional embedding stack

Chose direct optional declarations for @huggingface/transformers, onnxruntime-node, and sharp over relying on transformer transitive dependencies because pnpm may nest them where packages/core/dist/** cannot resolve them, silently degrading recall to FTS-only. Keep them out of regular dependencies so --omit=optional remains supported, and align versions with transformers so pnpm deduplicates to one stack. Validate resolution from a real subprocess rooted at the built worker; Vitest’s resolver can walk .pnpm and create a vacuous pass.