Key extractors by stable planet identity

PlanetField.gd

@@ -83,6 +83,7 @@ func _spawn_chunk_planet(chunk: Vector2i) -> void:
 	var planet: Node2D = planet_scene.instantiate()
 	planet.position = origin + data.local_pos
 	planet.planet_name = data.name
+	planet.planet_id = data.planet_id
 	planet.shape_seed = data.shape_seed
 	planet.definition_id = String(data.get("definition_id", "rocky"))
 	# Deferred: update_chunks() can run synchronously from inside a