Add PCB decoupling-proximity check from datasheet layout_rules.

PS-PLC-001 fires only with a LayoutGraph: 15 mm vs a 2 mm rule is an error, a 1 mm cap is silent, and a null millimetre uses the declared 3 mm default as WARNING.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/test_placement_check.py

@@ -0,0 +1,134 @@
+"""G2 placement vs datasheet layout_rules — only with a LayoutGraph.
+
+Favor: decoupling cap 15 mm from VDD pad with max_distance_mm=2 → PS-PLC-001;
+null mm uses declared 3 mm default as WARNING.
+Against: no PCB → silent; empty layout_rules skip; cap at 1 mm is ok;
+no cap on the net does not invent a millimetre.
+"""
+
+from __future__ import annotations
+
+from backend.pinscopex.models import (
+    Component,
+    ComponentConstraints,
+    ComponentType,
+    DesignGraph,
+    LayoutFootprint,
+    LayoutGraph,
+    LayoutPad,
+    Net,
+    NetType,
+    Pin,
+    PinConnection,
+)
+from backend.pinscopex.placement_check import check_placement
+
+
+def _graph():
+    u = Component(
+        reference="U1", value="", footprint="",
+        component_type=ComponentType.IC, mpn="UTEST",
+        pins={"1": "VDD", "2": "GND"},
+    )
+    c = Component(
+        reference="C1", value="100n", footprint="",
+        component_type=ComponentType.CAPACITOR, mpn="C",
+        pins={"1": "+3V3", "2": "GND"},
+    )
+    return DesignGraph(
+        components={"U1": u, "C1": c},
+        nets={
+            "+3V3": Net(name="+3V3", net_type=NetType.POWER, pins=[
+                PinConnection(component_ref="U1", pin_number="1"),
+                PinConnection(component_ref="C1", pin_number="1"),
+            ]),
+            "GND": Net(name="GND", net_type=NetType.GROUND, pins=[
+                PinConnection(component_ref="U1", pin_number="2"),
+                PinConnection(component_ref="C1", pin_number="2"),
+            ]),
+        },
+    )
+
+
+def _cons(max_mm: float | None):
+    return {
+        "UTEST": ComponentConstraints(
+            mpn="UTEST",
+            pintable=[Pin(number=1, name="VDD"), Pin(number=2, name="GND")],
+            absolute_maximum_ratings=[], rules=[],
+            layout_rules=[{
+                "kind": "decoupling_proximity",
+                "pin": "VDD",
+                "max_distance_mm": max_mm,
+                "source_page": 14,
+            }],
+        )
+    }
+
+
+def _layout(cap_x: float) -> LayoutGraph:
+    return LayoutGraph(
+        footprints={
+            "U1": LayoutFootprint(
+                reference="U1", x=0, y=0, layer="F.Cu",
+                pads=[
+                    LayoutPad(number="1", x=0.0, y=0.0, net="+3V3"),
+                    LayoutPad(number="2", x=0.0, y=1.0, net="GND"),
+                ],
+            ),
+            "C1": LayoutFootprint(
+                reference="C1", x=cap_x, y=0, layer="F.Cu",
+                pads=[
+                    LayoutPad(number="1", x=cap_x, y=0.0, net="+3V3"),
+                    LayoutPad(number="2", x=cap_x, y=0.5, net="GND"),
+                ],
+            ),
+        }
+    )
+
+
+def test_cap_15mm_from_2mm_rule_is_ps_plc_001():
+    findings = check_placement(_graph(), _cons(2.0), _layout(15.0))
+    assert len(findings) == 1
+    f = findings[0]
+    assert f.rule_id == "PS-PLC-001"
+    assert f.source == "placement_check"
+    assert f.status == "ERROR"
+    assert f.net == "+3V3"
+    assert "1" in (f.pins or [])
+    assert f.source_page == 14
+
+
+def test_cap_1mm_is_silent():
+    assert check_placement(_graph(), _cons(2.0), _layout(1.0)) == []
+
+
+def test_no_layout_graph_is_silent():
+    assert check_placement(_graph(), _cons(2.0), None) == []
+
+
+def test_empty_layout_rules_skip():
+    cons = {
+        "UTEST": ComponentConstraints(
+            mpn="UTEST",
+            pintable=[Pin(number=1, name="VDD"), Pin(number=2, name="GND")],
+            absolute_maximum_ratings=[], rules=[],
+            layout_rules=[],
+        )
+    }
+    assert check_placement(_graph(), cons, _layout(15.0)) == []
+
+
+def test_null_mm_uses_declared_3mm_default_as_warning():
+    findings = check_placement(_graph(), _cons(None), _layout(10.0))
+    assert len(findings) == 1
+    assert findings[0].status == "WARNING"
+    assert findings[0].rule_id == "PS-PLC-001"
+    assert "3 mm" in findings[0].finding or "3 mm" in (findings[0].why or "")
+
+
+def test_no_capacitor_does_not_invent_distance():
+    g = _graph()
+    g.components.pop("C1")
+    g.nets["+3V3"].pins = [PinConnection(component_ref="U1", pin_number="1")]
+    assert check_placement(g, _cons(2.0), _layout(15.0)) == []