Add golden ball interactable set piece to bridge room

- Add golden_ball_walk_target Node2D at position (1400, 550) on the grassy area above the bridge
- Add golden_ball SetPiece with polygon resource for interactable area
- Connect touched signal to _on_golden_ball_touched handler
- When player uses hand cursor, they walk to the target and narration reveals the golden ball
- Use ScriptBuilder.and_then() to chain walk_to_deferred and narrate actions

scenes/kq4_021_bridge_over_stream/golden_ball_polygon.tres.uid

@@ -0,0 +1 @@
+uid://c8k3m9xq2w5np