Add MMR re-ranking to hybrid recall

[bug-ops]

Parent: #740 (P1)

Problem

Hybrid search top-N results may contain semantically redundant entries, reducing effective recall diversity.

Solution

Add optional Maximal Marginal Relevance (MMR) post-processing in SemanticMemory::recall():

• score_mmr = λ * relevance - (1-λ) * max_similarity_to_selected
• Default λ = 0.7
• Config: memory.semantic.mmr_enabled (bool), memory.semantic.mmr_lambda (f32)

Affected crates

• zeph-memory

Acceptance criteria

• MMR re-ranking implemented and configurable
• Results are more diverse when enabled (test with near-duplicate embeddings)
• No performance regression when disabled