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/parser/vendors/dell/parser_test.go

@@ -152,6 +152,9 @@ func TestParseNestedTSRZip(t *testing.T) {
 	if got := result.Hardware.CPUs[0].Model; got != "Intel(R) Xeon(R) Gold 6330" {
 		t.Fatalf("unexpected cpu model: %q", got)
 	}
+	if got := result.Hardware.CPUs[0].SerialNumber; got != "ABCD" {
+		t.Fatalf("expected CPU serial fallback from PPIN, got %q", got)
+	}
 
 	if len(result.Hardware.NetworkAdapters) != 1 {
 		t.Fatalf("expected 1 network adapter, got %d", len(result.Hardware.NetworkAdapters))