Add cachedItems to kits; prevent kit item drops

Introduce an abstract cachedItems property on Kit (ConcurrentHashMap<UUID, List<ItemStack>>) to track per-player kit items. Update implementations (BackupKit, GoblinKit, IceMageKit, TemplateKit) to override and initialize cachedItems. GameStateListener now checks dropped items against the selected kit cache and cancels drops (playing a sound) for kit items, and adds handlers to cancel item pickup/despawn outside active game states. Add necessary imports for ItemStack, UUID, ConcurrentHashMap and new event types.

src/main/kotlin/club/mcscrims/speedhg/kit/Kit.kt

@@ -5,6 +5,9 @@ import club.mcscrims.speedhg.kit.ability.PassiveAbility
 import net.kyori.adventure.text.Component
 import org.bukkit.Material
 import org.bukkit.entity.Player
+import org.bukkit.inventory.ItemStack
+import java.util.UUID
+import java.util.concurrent.ConcurrentHashMap
 
 /**
  * Base class for every kit in SpeedHG.
@@ -61,6 +64,8 @@ abstract class Kit {
 	//  Item distribution
 	// -------------------------------------------------------------------------
 
+	abstract val cachedItems: ConcurrentHashMap<UUID, List<ItemStack>>
+
 	/**
 	 * Give the player their kit-specific items at game start (after teleportation).
 	 * The standard HG items (soup, compass, etc.) are already given by [GameManager].