Add reranker models #1568

Problem description

On-device RAG has embedders but no reranker.

Proposed solution

Add variants:

• mxbai-rerank-xsmall-v1 (EN, ~70M)
• Qwen3-Reranker-0.6B (multilingual, reuses the Qwen3 export)
• bge-reranker-v2-m3 (multilingual, 568M)

Benefits to React Native ExecuTorch

Better retrieval quality for RAG.