Hybrid search

Pure ANN misses exact keywords. Pure lexical misses paraphrases. Fuse both.

hits = docs.seek_hybrid(
    query=embed("alpha design note"),
    text="alpha design note",
    top=5,
    lexical_weight=0.25,   # default per docs/cpp/api-reference/vault.md
)

lexical_weight ∈ [0, 1]. 0 means pure vector; 1 means pure lexical. The fused score is on each SearchResult.