engine: seeded RNG helper, first tests

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.

internal/engine/rng.go

@@ -0,0 +1,16 @@
+// Package engine implements the agent-based spread model: build a social
+// network, run an independent cascade over it, and measure the effect of
+// education strategies. It is pure: no I/O, no web dependencies, and all
+// randomness flows from seeds in the config, so identical inputs always
+// produce identical runs.
+package engine
+
+import "math/rand/v2"
+
+// newRand returns a deterministic random stream for the given seed. The
+// engine never touches global random state; every source of randomness
+// (graph build, edge thresholds, education sampling) gets its own seeded
+// stream so each can be varied independently.
+func newRand(seed uint64) *rand.Rand {
+	return rand.New(rand.NewPCG(seed, 0))
+}