feat: add floor themes with status effect modifiers

Add 4 floor themes (Swamp/Volcano/Glacier/Inferno) that boost status
effect damage on matching floors. Realign boss patterns to match themes
and add PatternFreeze for the Glacier boss.

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

entity/monster.go

@@ -41,6 +41,7 @@ const (
 	PatternPoison              // applies poison
 	PatternBurn                // applies burn to random player
 	PatternHeal                // heals self
+	PatternFreeze              // applies freeze to all players
 )
 
 type Monster struct {