basic natural respawn

scripts/core/SaveState.cs

@@ -3,7 +3,12 @@ using System.Collections.Generic;
 
 public class SaveState
 {
+	// Permanent one-time removals (e.g. chests, unique items)
 	public HashSet<string> CollectedPickups { get; set; } = new();
+	
+	// Respawnables: pickupId -> unixTimeSeconds when it becomes available again
+	public Dictionary<string, long> PickupRespawnAt { get; set; } = new();
+	
 	//public Vector2I? LastPlayerChunk { get; set; } // optional debug, requires "using Godot;"
 	//public int[]? LastPlayerChunk { get; set; }  // [x,y] same debug as above, does not require Godot
 }