Preferential attachment via an attachment pool that holds one entry per
edge endpoint, so uniform draws are degree-proportional: that is the
whole hub-forming mechanism. A triangle step closes friend-of-a-friend
links with probability triangleProb, giving friend-group clustering.
Semantics ported from networkx, NOT its RNG stream (per the handoff,
no cross-language number matching).
Tests are property-based: size, edge bounds, connectivity, hub
formation across seeds, plus exact determinism for a fixed seed.
'go test ./...', gofmt and golangci-lint all clean.
