v0.5.1

• Adds Torus.Embeddings.Gemini to support Gemini embeddings.
• Extends semantic search docs on how to stack embedders
• Adds :distance_key option to Torus.semantic/5 to allow selecting distance key to the result map. Later on we'll rely on this to support hybrid search.
• Correctly swaps > and < operators for pre-filtering when changing order in Torus.semantic/5 search.