Restore build_placement_plan import in the PCB pipeline.

MODE=pcb classify crashed with NameError after the finding-engine import swap.

tests/test_pcb_review.py

@@ -537,3 +537,10 @@ def test_gnd_stitch_info_when_signal_has_pour_but_no_via():
     layout.vias = [LayoutVia(x=10, y=0, net="GND", drill=0.3)]
     assert check_pcb_gnd_stitch(graph, layout) == []
 
+
+def test_pcb_pipeline_imports_build_placement_plan():
+    import backend.services.pcb_pipeline as pcb_pipeline
+    from backend.periscopex.functional_groups import build_placement_plan as expected
+
+    assert pcb_pipeline.build_placement_plan is expected
+