payments/migration1: freeze lnwire and record dependency

Also freeze the lnwire and record packages used by the migration.

Copy the minimal subset of lnwire files (16) into
payments/db/migration1/lnwire/ and all record files (6) into
payments/db/migration1/record/. Three lnwire files are trimmed to avoid
pulling in the full message-type dispatch tree — all changes are purely
subtractive and can be verified with:

  diff lnwire/message.go payments/db/migration1/lnwire/message.go
  diff lnwire/writer.go  payments/db/migration1/lnwire/writer.go
  diff lnwire/lnwire.go  payments/db/migration1/lnwire/lnwire.go

All migration1 files now import only the frozen packages, removing the
live dependency on lnwire and record so future changes to those packages
cannot affect migration correctness.

payments/db/migration1/record/mpp.go

@@ -0,0 +1,112 @@
+package record
+
+import (
+	"fmt"
+	"io"
+
+	"github.com/lightningnetwork/lnd/payments/db/migration1/lnwire"
+	"github.com/lightningnetwork/lnd/tlv"
+)
+
+// MPPOnionType is the type used in the onion to reference the MPP fields:
+// total_amt and payment_addr.
+const MPPOnionType tlv.Type = 8
+
+// MPP is a record that encodes the fields necessary for multi-path payments.
+type MPP struct {
+	// paymentAddr is a random, receiver-generated value used to avoid
+	// collisions with concurrent payers.
+	paymentAddr [32]byte
+
+	// totalMsat is the total value of the payment, potentially spread
+	// across more than one HTLC.
+	totalMsat lnwire.MilliSatoshi
+}
+
+// NewMPP generates a new MPP record with the given total and payment address.
+func NewMPP(total lnwire.MilliSatoshi, addr [32]byte) *MPP {
+	return &MPP{
+		paymentAddr: addr,
+		totalMsat:   total,
+	}
+}
+
+// PaymentAddr returns the payment address contained in the MPP record.
+func (r *MPP) PaymentAddr() [32]byte {
+	return r.paymentAddr
+}
+
+// TotalMsat returns the total value of an MPP payment in msats.
+func (r *MPP) TotalMsat() lnwire.MilliSatoshi {
+	return r.totalMsat
+}
+
+// MPPEncoder writes the MPP record to the provided io.Writer.
+func MPPEncoder(w io.Writer, val interface{}, buf *[8]byte) error {
+	if v, ok := val.(*MPP); ok {
+		err := tlv.EBytes32(w, &v.paymentAddr, buf)
+		if err != nil {
+			return err
+		}
+
+		return tlv.ETUint64T(w, uint64(v.totalMsat), buf)
+	}
+	return tlv.NewTypeForEncodingErr(val, "MPP")
+}
+
+const (
+	// minMPPLength is the minimum length of a serialized MPP TLV record,
+	// which occurs when the truncated encoding of total_amt_msat takes 0
+	// bytes, leaving only the payment_addr.
+	minMPPLength = 32
+
+	// maxMPPLength is the maximum length of a serialized MPP TLV record,
+	// which occurs when the truncated encoding of total_amt_msat takes 8
+	// bytes.
+	maxMPPLength = 40
+)
+
+// MPPDecoder reads the MPP record to the provided io.Reader.
+func MPPDecoder(r io.Reader, val interface{}, buf *[8]byte, l uint64) error {
+	if v, ok := val.(*MPP); ok && minMPPLength <= l && l <= maxMPPLength {
+		if err := tlv.DBytes32(r, &v.paymentAddr, buf, 32); err != nil {
+			return err
+		}
+
+		var total uint64
+		if err := tlv.DTUint64(r, &total, buf, l-32); err != nil {
+			return err
+		}
+		v.totalMsat = lnwire.MilliSatoshi(total)
+
+		return nil
+	}
+	return tlv.NewTypeForDecodingErr(val, "MPP", l, maxMPPLength)
+}
+
+// Record returns a tlv.Record that can be used to encode or decode this record.
+func (r *MPP) Record() tlv.Record {
+	// Fixed-size, 32 byte payment address followed by truncated 64-bit
+	// total msat.
+	size := func() uint64 {
+		return 32 + tlv.SizeTUint64(uint64(r.totalMsat))
+	}
+
+	return tlv.MakeDynamicRecord(
+		MPPOnionType, r, size, MPPEncoder, MPPDecoder,
+	)
+}
+
+// PayloadSize returns the size this record takes up in encoded form.
+func (r *MPP) PayloadSize() uint64 {
+	return 32 + tlv.SizeTUint64(uint64(r.totalMsat))
+}
+
+// String returns a human-readable representation of the mpp payload field.
+func (r *MPP) String() string {
+	if r == nil {
+		return "<nil>"
+	}
+
+	return fmt.Sprintf("total=%v, addr=%x", r.totalMsat, r.paymentAddr)
+}