fix: detect duplicate ResourceNode node_id and use interaction_point for distance calc

- Push error and disable node when duplicate node_id is registered
  to prevent silent conflict during find_available()
- Use interaction_point.global_position instead of self.global_position
  in find_available() so distance is measured to the marker NPCs
  actually navigate toward

world/resource_nodes/ResourceNode.gd

@@ -24,6 +24,12 @@ var reserved_by: int = -1
 func _ready():
 	if node_id.is_empty():
 		push_error("ResourceNode at %s has empty node_id" % get_path())
+		enabled = false
+	else:
+		var existing := get_by_id(node_id)
+		if existing != null:
+			push_error("Duplicate ResourceNode node_id '%s' at %s; first registered at %s" % [node_id, get_path(), existing.get_path()])
+			enabled = false
 	_all.append(self)
 	add_to_group("resource_nodes")
 	amount_changed.connect(_update_debug_label)
@@ -93,7 +99,7 @@ static func find_available(action: StringName, agent_id: int, from_position: Vec
 			continue
 		if not node.is_available_for(agent_id):
 			continue
-		var dist := from_position.distance_squared_to(node.global_position)
+		var dist := from_position.distance_squared_to(node.interaction_point.global_position)
 		if dist < best_dist:
 			best_dist = dist
 			best = node