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

@@ -0,0 +1,8 @@
+[gd_resource type="Resource" script_class="PolygonPointsResource" format=3 uid="uid://c8k3m9xq2w5np"]
+
+[ext_resource type="Script" uid="uid://dtemboas3bi8y" path="res://PolygonPointsResource.gd" id="1_ppr"]
+
+[resource]
+script = ExtResource("1_ppr")
+points = PackedVector2Array(1350, 520, 1450, 520, 1450, 580, 1350, 580)
+metadata/_custom_type_script = "uid://dtemboas3bi8y"