Function compute_score 

pub fn compute_score(
    memory: &MemoryNode,
    query_tokens: &[&str],
    vector_similarity: f64,
    graph: &GraphEngine,
    bm25: &Bm25Index,
    now: DateTime<Utc>,
) -> ScoreBreakdown

Compute 9-component hybrid score for a memory against a query. The graph parameter is used to look up edge counts for graph strength scoring. The bm25 parameter provides BM25-based token overlap scoring; if the memory is in the index it uses the indexed score, otherwise falls back to score_text. The now parameter makes scoring deterministic and testable by avoiding internal clock reads.