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_test.go

@@ -0,0 +1,25 @@
+package models
+
+import "testing"
+
+func TestResolveCPUSerialNumber(t *testing.T) {
+	tests := []struct {
+		name   string
+		serial string
+		ppin   string
+		want   string
+	}{
+		{name: "serial has priority", serial: "CPU-SERIAL", ppin: "D46E5D6B1D3E40E1", want: "CPU-SERIAL"},
+		{name: "PPIN fallback", ppin: " D46E5D6B1D3E40E1 ", want: "D46E5D6B1D3E40E1"},
+		{name: "placeholder serial falls back", serial: "Unknown", ppin: "D44F8D6B9155EE0E", want: "D44F8D6B9155EE0E"},
+		{name: "placeholder PPIN rejected", ppin: "N/A", want: ""},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := ResolveCPUSerialNumber(tt.serial, tt.ppin); got != tt.want {
+				t.Fatalf("ResolveCPUSerialNumber(%q, %q) = %q, want %q", tt.serial, tt.ppin, got, tt.want)
+			}
+		})
+	}
+}