feat: add elite monsters with 5 prefix types

Elite monsters have ~20% spawn chance with Venomous, Burning, Freezing,
Bleeding, or Vampiric prefixes. Each prefix scales HP/ATK and applies
on-hit status effects (or life drain for Vampiric).

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

entity/monster.go

@@ -49,6 +49,8 @@ type Monster struct {
 	HP, MaxHP   int
 	ATK, DEF    int
 	IsBoss      bool
+	IsElite     bool
+	ElitePrefix ElitePrefixType
 	TauntTarget bool
 	TauntTurns  int
 	Pattern     BossPattern