Replace PyNNDescent with sqlite-vec when it adds ANN indexes, float16, and metadata filtering

[lsorber]

Sqlite-vec looks like a promising project for vector search with SQLite. We'd need the following features for RAGLite before we can switch from PyNNDescent to sqlite-vec:

• ANN indexes: Tracking issue: ANN (Approximate Nearest Neighbors) Index asg017/sqlite-vec#25
• Half-precision floating point embeddings: float16 and bfloat16 support asg017/sqlite-vec#27
• Metadata filtering: Tracking issue: metadata filtering asg017/sqlite-vec#26