feat(ts-sdk): add FastEmbed embedding provider

Summary

The Python SDK supports FastEmbed as an embedding provider, but the TypeScript OSS SDK (mem0ai/oss) does not. Add it to bring the TS SDK to parity.

Python reference	mem0/embeddings/fastembed.py
Registered in (Python)	mem0/utils/factory.py (EmbedderFactory)
Target file (TypeScript)	mem0-ts/src/oss/src/embeddings/fastembed.ts
Suggested implementation	Use the fastembed npm package (ONNX local embeddings).

Requirements

• Implement FastEmbedEmbedder in mem0-ts/src/oss/src/embeddings/fastembed.ts, extending Embedder (mem0-ts/src/oss/src/embeddings/base.ts) and mirroring the Python provider's behavior (embed / embedBatch).
• Register the "fastembed" provider in mem0-ts/src/oss/src/utils/factory.ts (EmbedderFactory).
• Add config typing in mem0-ts/src/oss/src/types/.
• Add a unit test under mem0-ts/src/oss/src/tests/.
• Add fastembed to mem0-ts/package.json (optional/peer dependency, lazy-imported like other providers).
• Update docs under docs/ if this provider is user-facing.

Reference pattern

Mirror an existing TS provider: embeddings/openai.ts.

Notes

fastembed (v2.x) is the JS port of Qdrant's FastEmbed — local/offline embeddings. Mirror the default model in mem0/embeddings/fastembed.py.

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Part of the TypeScript ↔ Python SDK provider-parity effort. One provider per issue (atomic).