feat(sim): add deterministic RNG and crash curve

Seed expansion uses SplitMix64 so all 32 seed bytes affect the stream;
copying the seed directly into xoshiro state left the first draw
dependent only on bytes 8-15.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

pkg/sim/rng_test.go

@@ -0,0 +1,51 @@
+package sim
+
+import "testing"
+
+func TestRNGIsDeterministic(t *testing.T) {
+	var seed [32]byte
+	copy(seed[:], "quantum-arcade-test-seed")
+	a, b := NewRNG(seed), NewRNG(seed)
+	for i := 0; i < 1000; i++ {
+		if x, y := a.Uint64(), b.Uint64(); x != y {
+			t.Fatalf("iteration %d: %d != %d", i, x, y)
+		}
+	}
+}
+
+func TestDifferentSeedsDiverge(t *testing.T) {
+	var s1, s2 [32]byte
+	copy(s1[:], "seed-one")
+	copy(s2[:], "seed-two")
+	a, b := NewRNG(s1), NewRNG(s2)
+	same := 0
+	for i := 0; i < 100; i++ {
+		if a.Uint64() == b.Uint64() {
+			same++
+		}
+	}
+	if same > 1 {
+		t.Fatalf("streams collided %d times in 100 draws", same)
+	}
+}
+
+func TestZeroSeedDoesNotDegenerate(t *testing.T) {
+	var seed [32]byte // all zeros
+	r := NewRNG(seed)
+	first := r.Uint64()
+	if first == 0 && r.Uint64() == 0 {
+		t.Fatal("zero seed produced a degenerate all-zero stream")
+	}
+}
+
+func TestUnitInRange(t *testing.T) {
+	var seed [32]byte
+	seed[0] = 9
+	r := NewRNG(seed)
+	for i := 0; i < 10000; i++ {
+		u := r.Unit()
+		if u < 0 || u >= 1<<32 {
+			t.Fatalf("Unit() = %v out of [0,1)", u)
+		}
+	}
+}