feat: hhtl_cascade_search score_fn callback — LEAF level pluggable score_fn(row, col) -> f32 replaces the hardcoded 1.0 placeholder. lance-graph passes LanceDB VectorSearch as the LEAF backend. ndarray provides the cascade logic. lance-graph provides the data. 23 p64 tests passing. https://claude.ai/code/session_01M3at4EuHVvQ8S95mSnKgtK#73