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/collector/redfish.go

@@ -3311,6 +3311,7 @@ func parseCPUs(docs []map[string]interface{}) []models.CPU {
 		if serial == "" && publicSerial == "" {
 			serial = findFirstNormalizedStringByKeys(doc, "SerialNumber")
 		}
+		ppin := firstNonEmpty(findFirstNormalizedStringByKeys(doc, "PPIN", "ProtectedIdentificationNumber"), publicSerial)
 		cpus = append(cpus, models.CPU{
 			Socket:       socket,
 			Model:        firstNonEmpty(asString(doc["Model"]), asString(doc["Name"])),
@@ -3318,8 +3319,8 @@ func parseCPUs(docs []map[string]interface{}) []models.CPU {
 			Threads:      asInt(doc["TotalThreads"]),
 			FrequencyMHz: int(redfishFirstNumeric(doc, "OperatingSpeedMHz", "CurrentSpeedMHz", "FrequencyMHz")),
 			MaxFreqMHz:   int(redfishFirstNumeric(doc, "MaxSpeedMHz", "TurboEnableMaxSpeedMHz", "TurboDisableMaxSpeedMHz")),
-			PPIN:         firstNonEmpty(findFirstNormalizedStringByKeys(doc, "PPIN", "ProtectedIdentificationNumber"), publicSerial),
-			SerialNumber: serial,
+			PPIN:         ppin,
+			SerialNumber: models.ResolveCPUSerialNumber(serial, ppin),
 			L1CacheKB:    l1,
 			L2CacheKB:    l2,
 			L3CacheKB:    l3,

internal/collector/redfish_test.go

@@ -1557,8 +1557,8 @@ func TestParseCPU_UsesPublicSerialAsPPINAndCurrentSpeedMHz(t *testing.T) {
 	if cpus[0].PPIN != "6FB5241E81CECDFD" {
 		t.Fatalf("expected PPIN from Oem.Public.SerialNumber, got %+v", cpus[0])
 	}
-	if cpus[0].SerialNumber != "" {
-		t.Fatalf("expected empty CPU serial number when only Public serial exists, got %+v", cpus[0])
+	if cpus[0].SerialNumber != "6FB5241E81CECDFD" {
+		t.Fatalf("expected CPU serial number to fall back to PPIN, got %+v", cpus[0])
 	}
 	if cpus[0].FrequencyMHz != 2700 {
 		t.Fatalf("expected CPU frequency from Oem.Public.CurrentSpeedMHz, got %+v", cpus[0])