engine: benchmark baseline (HolmeKim, RunScenario)

Manual baseline, not a CI gate: go test -bench=. -benchmem.
First numbers on a Ryzen 7 5800X: HolmeKim ~25us/op (48KB, 671
allocs), full RunScenario ~45us/op (78KB, 681 allocs). Plenty fast
until milestone 5 runs thousands of cascades per request; this is the
'before' picture for that work.
b.Loop() (Go 1.24+) replaces the old 'for i := 0; i < b.N; i++'
pattern and prevents the compiler optimising the loop body away.

internal/engine/bench_test.go

@@ -0,0 +1,30 @@
+package engine
+
+import "testing"
+
+// Benchmarks are a baseline, not a gate: run them manually with
+//
+//	go test -bench=. -benchmem ./internal/engine/
+//
+// The numbers become interesting in milestone 5, when the optimisation
+// feature starts running thousands of cascades per request; comparing
+// against this baseline catches accidental quadratic behaviour.
+
+func BenchmarkHolmeKim(b *testing.B) {
+	config := DefaultConfig()
+	for b.Loop() {
+		_, err := HolmeKim(config.NumStudents, config.EdgesPerNode, config.TriangleProb, newRand(config.GraphSeed))
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
+func BenchmarkRunScenario(b *testing.B) {
+	config := DefaultConfig()
+	for b.Loop() {
+		if _, err := RunScenario(config, StrategyMostConnected); err != nil {
+			b.Fatal(err)
+		}
+	}
+}