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/models/cpu_identity.go

@@ -0,0 +1,23 @@
+package models
+
+import "strings"
+
+// ResolveCPUSerialNumber returns a source-backed processor inventory identity.
+// A vendor-provided serial number has priority; PPIN is the documented CPU
+// serial identity and is used when a separate serial number is unavailable.
+func ResolveCPUSerialNumber(serialNumber, ppin string) string {
+	if serial := validCPUIdentity(serialNumber); serial != "" {
+		return serial
+	}
+	return validCPUIdentity(ppin)
+}
+
+func validCPUIdentity(value string) string {
+	value = strings.TrimSpace(value)
+	switch strings.ToLower(value) {
+	case "", "-", "n/a", "na", "none", "null", "unknown", "not available", "to be filled by o.e.m.":
+		return ""
+	default:
+		return value
+	}
+}