feat: add secret rooms and mini-bosses on floors 4/9/14/19

Add RoomSecret (5% chance) and RoomMiniBoss room types. Add 4 mini-boss
monsters at 60% of boss stats (Guardian's Herald, Warden's Shadow,
Overlord's Lieutenant, Archlich's Harbinger) with IsMiniBoss flag and
boss pattern logic. Secret rooms grant double treasure. Mini-boss rooms
are placed on floors 4/9/14/19 at room index 1.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

dungeon/generator_test.go

@@ -37,7 +37,49 @@ func TestRoomTypeProbability(t *testing.T) {
 	}
 	combatPct := float64(counts[RoomCombat]) / float64(n) * 100
 	if combatPct < 40 || combatPct > 50 {
-		t.Errorf("Combat room probability: got %.1f%%, want ~45%%", combatPct)
+		t.Errorf("Combat room probability: got %.1f%%, want ~45%% (range 5-50)", combatPct)
+	}
+}
+
+func TestSecretRoomInRandomType(t *testing.T) {
+	counts := make(map[RoomType]int)
+	n := 10000
+	for i := 0; i < n; i++ {
+		counts[RandomRoomType()]++
+	}
+	secretPct := float64(counts[RoomSecret]) / float64(n) * 100
+	if secretPct < 2 || secretPct > 8 {
+		t.Errorf("Secret room probability: got %.1f%%, want ~5%%", secretPct)
+	}
+	// Verify RoomMiniBoss is never returned by RandomRoomType
+	if counts[RoomMiniBoss] > 0 {
+		t.Errorf("MiniBoss rooms should not be generated randomly, got %d", counts[RoomMiniBoss])
+	}
+}
+
+func TestMiniBossRoomPlacement(t *testing.T) {
+	for _, floorNum := range []int{4, 9, 14, 19} {
+		floor := GenerateFloor(floorNum)
+		found := false
+		for _, r := range floor.Rooms {
+			if r.Type == RoomMiniBoss {
+				found = true
+				break
+			}
+		}
+		if !found {
+			t.Errorf("Floor %d should have a mini-boss room", floorNum)
+		}
+	}
+	// Non-miniboss floors should not have mini-boss rooms
+	for _, floorNum := range []int{1, 3, 5, 10} {
+		floor := GenerateFloor(floorNum)
+		for _, r := range floor.Rooms {
+			if r.Type == RoomMiniBoss {
+				t.Errorf("Floor %d should not have a mini-boss room", floorNum)
+				break
+			}
+		}
 	}
 }