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/server/device_repository.go

@@ -57,7 +57,7 @@ func BuildHardwareDevices(hw *models.HardwareConfig) []models.HardwareDevice {
 			Source:           "cpus",
 			Slot:             fmt.Sprintf("CPU%d", cpu.Socket),
 			Model:            cpu.Model,
-			SerialNumber:     cpu.SerialNumber,
+			SerialNumber:     models.ResolveCPUSerialNumber(cpu.SerialNumber, cpu.PPIN),
 			Cores:            cpu.Cores,
 			Threads:          cpu.Threads,
 			FrequencyMHz:     cpu.FrequencyMHz,