feat(models): resolve CPU serial from source PPIN as common fallback

Centralize CPU identity in models.ResolveCPUSerialNumber: an explicit
source serial wins, otherwise a valid source PPIN is used, placeholders
rejected. Dell, H3C and Redfish apply it while parsing; canonical-device
and Reanimator conversion apply it again at the output boundary. No
identity is synthesized from socket/model/board serial.

See ADL-058.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/exporter/reanimator_converter_test.go

@@ -320,6 +320,31 @@ func TestConvertToReanimator_CPUSerialIsNotSynthesizedAndSocketIsDeduped(t *test
 	}
 }
 
+func TestConvertToReanimator_CPUSerialUsesPPINFallback(t *testing.T) {
+	input := &models.AnalysisResult{
+		Hardware: &models.HardwareConfig{
+			BoardInfo: models.BoardInfo{SerialNumber: "BOARD-001"},
+			Devices: []models.HardwareDevice{{
+				Kind:  models.DeviceKindCPU,
+				Slot:  "CPU0",
+				Model: "Intel Xeon",
+				Details: map[string]any{
+					"socket": 0,
+					"ppin":   "D46E5D6B1D3E40E1",
+				},
+			}},
+		},
+	}
+
+	out, err := ConvertToReanimator(input)
+	if err != nil {
+		t.Fatalf("ConvertToReanimator() failed: %v", err)
+	}
+	if len(out.Hardware.CPUs) != 1 || out.Hardware.CPUs[0].SerialNumber != "D46E5D6B1D3E40E1" {
+		t.Fatalf("expected CPU serial fallback from PPIN, got %+v", out.Hardware.CPUs)
+	}
+}
+
 func TestConvertToReanimator_ExportsEventLogsAndOmitsPCIeBDFJSON(t *testing.T) {
 	input := &models.AnalysisResult{
 		Filename:    "events.json",