engine: additive forward probability, behaviour-preserving

Forwarding was a single global ForwardProb; make it a per-student
composite (Config.ForwardChance): baseline propensity raised by the
fake's Novelty, lowered by ambient HarmAwareness, and scaled down for an
educated student by ProgramEffect (1 = today's hard block). RunCascade
now takes a precomputed per-student []float64 chance and has no education
special case. Defaults are behaviour-neutral, so the 82/58/6 golden
tests are unchanged; the model is tuned in a later slice.

internal/engine/forwardchance_test.go

@@ -0,0 +1,67 @@
+package engine
+
+import "testing"
+
+// ForwardChance is the additive composite the whole model rests on. These
+// pin the formula's shape (which lever pushes which way, the clamp, and how
+// the program scales an educated student) in terms of the weight constants,
+// so tuning the weights later does not silently break the relationships.
+func TestForwardChance(t *testing.T) {
+	base := DefaultConfig() // ForwardProb 0.38, novelty/harm 0, programEffect 1
+
+	t.Run("default not educated is the baseline", func(t *testing.T) {
+		if got := base.ForwardChance(false); got != base.ForwardProb {
+			t.Errorf("ForwardChance(false) = %v, want baseline %v", got, base.ForwardProb)
+		}
+	})
+
+	t.Run("default educated never forwards under a full program", func(t *testing.T) {
+		if got := base.ForwardChance(true); got != 0 {
+			t.Errorf("ForwardChance(true) = %v, want 0 (programEffect 1)", got)
+		}
+	})
+
+	t.Run("novelty raises forwarding", func(t *testing.T) {
+		config := base
+		config.Novelty = 1
+		want := base.ForwardProb + noveltyWeight
+		if got := config.ForwardChance(false); got != want {
+			t.Errorf("ForwardChance = %v, want %v", got, want)
+		}
+	})
+
+	t.Run("harm awareness lowers forwarding", func(t *testing.T) {
+		config := base
+		config.HarmAwareness = 0.5
+		want := base.ForwardProb - harmAwarenessWeight*0.5
+		if got := config.ForwardChance(false); got != want {
+			t.Errorf("ForwardChance = %v, want %v", got, want)
+		}
+	})
+
+	t.Run("clamps to zero", func(t *testing.T) {
+		config := base
+		config.HarmAwareness = 1 // 0.38 - 0.40 < 0
+		if got := config.ForwardChance(false); got != 0 {
+			t.Errorf("ForwardChance = %v, want clamped 0", got)
+		}
+	})
+
+	t.Run("clamps to the ceiling", func(t *testing.T) {
+		config := base
+		config.ForwardProb = 0.9
+		config.Novelty = 1 // 0.9 + 0.30 > 0.95
+		if got := config.ForwardChance(false); got != maxForwardChance {
+			t.Errorf("ForwardChance = %v, want clamped %v", got, maxForwardChance)
+		}
+	})
+
+	t.Run("a softer program leaves some forwarding", func(t *testing.T) {
+		config := base
+		config.ProgramEffect = 0.5
+		want := base.ForwardProb * 0.5
+		if got := config.ForwardChance(true); got != want {
+			t.Errorf("ForwardChance(true) = %v, want %v", got, want)
+		}
+	})
+}