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_test.go

@@ -315,3 +315,18 @@ func TestHandleGetConfig_ReturnsCanonicalHardware(t *testing.T) {
 		t.Fatalf("did not expect legacy hardware.cpus in config response")
 	}
 }
+
+func TestBuildHardwareDevices_CPUUsesPPINAsSerialFallback(t *testing.T) {
+	hw := &models.HardwareConfig{CPUs: []models.CPU{{Socket: 0, Model: "Intel Xeon", PPIN: "D46E5D6B1D3E40E1"}}}
+
+	devices := BuildHardwareDevices(hw)
+	for _, device := range devices {
+		if device.Kind == models.DeviceKindCPU {
+			if device.SerialNumber != "D46E5D6B1D3E40E1" {
+				t.Fatalf("expected CPU serial fallback from PPIN, got %q", device.SerialNumber)
+			}
+			return
+		}
+	}
+	t.Fatal("CPU device was not built")
+}