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>

combat/combat.go

@@ -82,8 +82,8 @@ func AttemptFlee(fleeChance float64) bool {
 }
 
 func MonsterAI(m *entity.Monster, players []*entity.Player, turnNumber int) (targetIdx int, isAoE bool) {
-	if m.IsBoss && turnNumber > 0 && turnNumber%3 == 0 {
-		return -1, true // AoE every 3 turns for all bosses
+	if (m.IsBoss || m.IsMiniBoss) && turnNumber > 0 && turnNumber%3 == 0 {
+		return -1, true // AoE every 3 turns for all bosses and mini-bosses
 	}
 	if m.TauntTarget {
 		for i, p := range players {