Apply the finding engine to schematic and PCB reviews.

FACT, REQUIREMENT, and INFERENCE are separate fields; datasheet provenance
lives in the rule DB so Recommended cannot stay ERROR; LLM output is REVIEW.
Designer decisions persist across rescans. Schematic and PCB share one library
and the same finding object in the report UI.

tests/test_finding_engine.py

@@ -0,0 +1,135 @@
+"""Finding engine — FACT/REQUIREMENT/INFERENCE, provenance clamp, decisions."""
+
+from __future__ import annotations
+
+from backend.periscopex.finding_engine import (
+    DesignerDecision,
+    apply_decisions,
+    complete_finding,
+    finding_fingerprint,
+    lookup_rule,
+)
+from backend.periscopex.models import Finding
+from backend.periscopex.validate import assign_finding_ids, _parse_review
+
+
+def test_recommended_never_error():
+    f = Finding(
+        designator="U1",
+        finding="layout note",
+        why="place close",
+        status="ERROR",
+        rule_id="PE-THM-001",
+        source="pcb_power_thermal",
+        evidence_status="SUFFICIENT",
+        facts="no pour",
+        requirement="datasheet layout page",
+    )
+    complete_finding(f)
+    assert f.provenance == "RECOMMENDED"
+    assert f.status != "ERROR"
+    assert f.finding_class != "RULE"
+
+
+def test_llm_review_is_never_rule():
+    result = _parse_review(
+        {
+            "findings": [{
+                "finding": "strap",
+                "why": "datasheet",
+                "status": "ERROR",
+                "source_page": 2,
+                "source_quote": "PSEL must be high for 3.3 V.",
+                "recommendation": "Tie PSEL high.",
+            }],
+            "checked_areas": [],
+        },
+        "U1",
+        "PART",
+    )
+    f = result.findings[0]
+    complete_finding(f)
+    assert f.finding_class == "REVIEW"
+    assert f.facts
+    assert f.requirement
+    assert f.status == "ERROR"
+
+
+def test_insufficient_evidence_does_not_stay_error():
+    f = Finding(
+        designator="U1",
+        finding="narrow trace",
+        why="",
+        status="ERROR",
+        source="pcb_power_thermal",
+        rule_id="PE-PWR-001",
+        evidence_status="INSUFFICIENT",
+    )
+    complete_finding(f)
+    assert f.status != "ERROR"
+    assert "Insufficient evidence" in f.inference
+    assert f.finding_class != "RULE" or f.evidence_status == "INSUFFICIENT"
+
+
+def test_decision_suppresses_rescan_nag():
+    f = Finding(
+        designator="U1",
+        finding="PSEL low",
+        why="must be high",
+        status="ERROR",
+        rule_id="PE-MUX-001",
+        source="pin_mux_check",
+        net="PSEL",
+        aspect="config",
+        facts="PSEL net is GND",
+        requirement="PSEL high for 3V3",
+    )
+    complete_finding(f)
+    assert f.status == "ERROR"
+    d = DesignerDecision(
+        decision_id="DEC-1",
+        fingerprint=finding_fingerprint(f),
+        rule_id="PE-MUX-001",
+        designator="U1",
+        net="PSEL",
+        aspect="config",
+        intent="wontfix",
+        reason="PSEL low intentional",
+    )
+    apply_decisions([f], [d])
+    assert f.suppressed is True
+    assert f.status == "INFO"
+    assert f.finding_class == "INFO"
+    assert "PSEL low intentional" in f.inference
+
+
+def test_schematic_assign_ids_fills_engine_fields():
+    f = Finding(
+        designator="U3",
+        finding="NC tied",
+        why="NC must float",
+        status="ERROR",
+        rule_id="PE-NC-001",
+        source="nc_pin_check",
+    )
+    assign_finding_ids([f])
+    assert f.finding_id == "U3-001"
+    assert f.facts == "NC tied"
+    assert f.requirement
+    assert f.provenance == "MANDATORY"
+    assert f.finding_class == "RULE"
+    assert f.confidence is not None
+
+
+def test_same_object_pcb_and_schema():
+    assert lookup_rule("PE-NC-001").domain == "schema"
+    assert lookup_rule("PE-LAY-001").domain == "pcb"
+    sch = Finding(designator="U1", finding="x", status="INFO", rule_id="PE-NC-001", source="nc_pin_check")
+    pcb = Finding(designator="U1", finding="y", status="INFO", rule_id="PE-LAY-001", source="pcb_net_match")
+    complete_finding(sch)
+    complete_finding(pcb)
+    for obj in (sch, pcb):
+        assert obj.facts
+        assert obj.finding_class
+        assert obj.provenance
+        assert obj.evidence_status