rng.go is a construction point, not an abstraction wall; components
take *rand.Rand directly (idiomatic Go dependency injection) and only
RunScenario creates streams, from config seeds. Decided 2026-06-10
over the alternatives (interface wall, folding newRand into
scenario.go).
math/rand/v2 (Go 1.22+) replaces the old math/rand: PCG generator,
no global Seed(), and rand.New(rand.NewPCG(seed, 0)) gives an isolated
deterministic stream. Each randomness consumer (graph, thresholds,
education sampling) will get its own stream so levers vary independently.
Tests live next to the code as *_test.go; 'go test ./...' runs them all.
'for i := range 100' is Go 1.22 range-over-int.
