relationshipSignature rebuilt a node map and scanned every graph edge
for each side of each matched pair, so comparison cost grew as
pairs x (nodes + edges). Build node and adjacency indexes once per
graph in ItemContext; signatures, sorting and output are unchanged.
At 4000 matched modules, end-to-end compare: 5.7s -> 2.9s (2 edges
per node) and 10.4s -> 5.8s (6 edges per node); 8000: 3.3x and 2.5x.
Output JSON is byte-identical on the base commit.
keemsisi