Function graph_strength_for_memory 

pub fn graph_strength_for_memory(graph: &GraphEngine, memory_id: &str) -> f64

Compute graph strength for a memory node by combining raw graph metrics.

Uses PageRank, betweenness centrality, connectivity, and edge weights from the memory’s code-graph neighbors to produce a 0.0-1.0 score. Weights: PageRank 40%, betweenness 30%, connectivity 20%, edge weight 10%.