feat: carve Jajce river into terrain

tests/jajce_world_scaffold_test.gd

@@ -1,5 +1,7 @@
 extends SceneTree
 
+const JajceWatercourse := preload("res://world/jajce/JajceWatercourse.gd")
+
 var failures: Array[String] = []
 
 
@@ -213,7 +215,7 @@ func _run() -> void:
 		"Waterfall mist particles should have visible billboard geometry"
 	)
 	_check(
-		waterfall_mist.position.distance_to(Vector3(25.0, 2.05, -24.0)) < 0.01,
+		waterfall_mist.position.distance_to(Vector3(25.0, 0.65, -21.5)) < 0.01,
 		"Waterfall mist should stay at the waterfall base instead of obscuring the village"
 	)
 	_check(world.has_node("WaterRoot/RiverSurface"), "WaterRoot should include a river surface")
@@ -222,10 +224,34 @@ func _run() -> void:
 		river_ribbon.mesh is ArrayMesh,
 		"River should build a lightweight ribbon from the current Terrain3D surface"
 	)
+	var upper_river_ribbon := (
+		world.get_node("WaterRoot/RiverSurface/UpperRiverRibbon") as MeshInstance3D
+	)
+	_check(
+		upper_river_ribbon.mesh is ArrayMesh,
+		"Waterfall shelf should expose a terrain-following upper stream"
+	)
 	_check(
 		world.has_node("WaterRoot/RiverSurface/PlungePool"),
 		"River should widen into a readable plunge pool beneath the waterfall"
 	)
+	var plunge_pool := world.get_node("WaterRoot/RiverSurface/PlungePool") as MeshInstance3D
+	var expected_pool_height: float = terrain.data.get_height(
+		Vector3(JajceWatercourse.PLUNGE_POOL_CENTER.x, 0.0, JajceWatercourse.PLUNGE_POOL_CENTER.y)
+	)
+	_check(
+		(
+			absf(
+				(
+					plunge_pool.global_position.y
+					- expected_pool_height
+					- JajceWatercourse.WATER_SURFACE_OFFSET
+				)
+			)
+			< 0.03
+		),
+		"Plunge-pool water should follow the carved basin height"
+	)
 	var environment: Environment = (
 		(world.get_node("WorldEnvironment") as WorldEnvironment).environment
 	)