test: harden jajce terrain navigation

tests/jajce_world_scaffold_test.gd

@@ -26,6 +26,12 @@ func _run() -> void:
 		terrain.data_directory == "res://terrain/jajce/data",
 		"Terrain3D should use dedicated Jajce data"
 	)
+	_check(terrain.collision_mode != 0, "Terrain3D collision should be enabled for gameplay")
+	var greybox_ground := world.get_node("NavigationRegion3D/GreyboxGround") as StaticBody3D
+	_check(
+		greybox_ground.collision_layer == 0 and greybox_ground.collision_mask == 0,
+		"GreyboxGround should not provide runtime physics collision"
+	)
 	_check(
 		configured_texture_count == 6,
 		(
@@ -215,6 +221,9 @@ func _run() -> void:
 				not path.is_empty(),
 				"Jajce navigation path should exist from %s to %s" % [origin, destination]
 			)
+			_check_path_tracks_terrain(
+				terrain, path, "Jajce navigation path should track Terrain3D height"
+			)
 
 	var adapter := ActiveWorldAdapter.new()
 	adapter.pantry_storage = pantry
@@ -304,3 +313,20 @@ func _wait_for_navigation_map(navigation_map: RID) -> void:
 func _check(condition: bool, message: String) -> void:
 	if not condition:
 		failures.append(message)
+
+
+func _check_path_tracks_terrain(terrain: Terrain3D, path: PackedVector3Array, message: String) -> void:
+	if path.is_empty():
+		return
+
+	for point in path:
+		var terrain_height: float = terrain.data.get_height(point)
+		if is_nan(terrain_height):
+			failures.append("%s: missing terrain height at %s" % [message, point])
+			return
+		if absf(point.y - terrain_height) > 0.85:
+			failures.append(
+				"%s: path point %s is too far from terrain height %.2f"
+				% [message, point, terrain_height]
+			)
+			return