payments/migration1: freeze core payment code

Copy the core payments/db code into payments/db/migration1 and
add the required sqlc-generated types/queries from sqldb/sqlc.
This effectively freezes the migration code so it stays robust
against future query or schema changes in the main payments package.

Replace the delegation to channeldb.ReadElement/WriteElement with
self-contained, frozen implementations that only handle the exact types
required by this migration package. This removes the dependency on the
live channeldb codec so that future changes to channeldb serialization
cannot silently corrupt or break the migration.

UnknownElementType is also defined locally for the same reason.

payments/db/migration1/route.go

@@ -0,0 +1,138 @@
+package migration1
+
+import (
+	"fmt"
+
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/lightningnetwork/lnd/lnwire"
+	"github.com/lightningnetwork/lnd/record"
+	"github.com/lightningnetwork/lnd/tlv"
+)
+
+// Vertex is a frozen local type representing a 33-byte compressed public key,
+// equivalent to route.Vertex at the time this migration was written.
+type Vertex [33]byte
+
+// NewVertex returns a new Vertex from a compressed public key.
+func NewVertex(pub *btcec.PublicKey) Vertex {
+	var v Vertex
+	copy(v[:], pub.SerializeCompressed())
+
+	return v
+}
+
+// NewVertexFromBytes returns a new Vertex from a serialized compressed public
+// key byte slice.
+func NewVertexFromBytes(b []byte) (Vertex, error) {
+	if len(b) != 33 {
+		return Vertex{}, fmt.Errorf("invalid vertex length %d, "+
+			"expected 33", len(b))
+	}
+
+	var v Vertex
+	copy(v[:], b)
+
+	return v, nil
+}
+
+// Hop is a frozen local snapshot of route.Hop, containing exactly the fields
+// that existed at the time this migration was written. This ensures the
+// migration's serialization boundary is independent of future changes to
+// route.Hop, in particular the planned removal of LegacyPayload once the KV
+// backend is phased out.
+type Hop struct {
+	// PubKeyBytes is the raw bytes of the public key of the target node.
+	PubKeyBytes Vertex
+
+	// ChannelID is the unique channel ID for the channel.
+	ChannelID uint64
+
+	// OutgoingTimeLock is the timelock value that should be used when
+	// crafting the _outgoing_ HTLC from this hop.
+	OutgoingTimeLock uint32
+
+	// AmtToForward is the amount that this hop will forward to the next
+	// hop.
+	AmtToForward lnwire.MilliSatoshi
+
+	// MPP encapsulates the data required for option_mpp. This field should
+	// only be set for the final hop.
+	MPP *record.MPP
+
+	// AMP encapsulates the data required for option_amp. This field should
+	// only be set for the final hop.
+	AMP *record.AMP
+
+	// CustomRecords if non-nil are a set of additional TLV records that
+	// should be included in the forwarding instructions for this node.
+	CustomRecords record.CustomSet
+
+	// LegacyPayload signals that this node doesn't understand the new TLV
+	// payload, so the legacy payload format must be used.
+	//
+	// NOTE: This field is preserved here even though it is marked for
+	// removal in the live route.Hop, because old KV data may have been
+	// serialized with LegacyPayload=true and the migration must be able to
+	// deserialize it correctly.
+	LegacyPayload bool
+
+	// Metadata is additional data sent along with the payment to the
+	// payee.
+	Metadata []byte
+
+	// EncryptedData is an encrypted data blob included for hops that are
+	// part of a blinded route.
+	EncryptedData []byte
+
+	// BlindingPoint is an ephemeral public key used by introduction nodes
+	// in blinded routes.
+	BlindingPoint *btcec.PublicKey
+
+	// TotalAmtMsat is the total amount for a blinded payment. This field
+	// should only be set for the final hop in a blinded path.
+	TotalAmtMsat lnwire.MilliSatoshi
+}
+
+// Route is a frozen local snapshot of route.Route, containing exactly the
+// fields that existed at the time this migration was written.
+type Route struct {
+	// TotalTimeLock is the cumulative (final) time lock across the entire
+	// route.
+	TotalTimeLock uint32
+
+	// TotalAmount is the total amount of funds required to complete a
+	// payment over this route, including fees.
+	TotalAmount lnwire.MilliSatoshi
+
+	// SourcePubKey is the pubkey of the node where this route originates.
+	SourcePubKey Vertex
+
+	// Hops contains details concerning the specific forwarding details at
+	// each hop.
+	Hops []*Hop
+
+	// FirstHopAmount is the amount that should actually be sent to the
+	// first hop. Only differs from TotalAmount for custom channels.
+	FirstHopAmount tlv.RecordT[
+		tlv.TlvType0, tlv.BigSizeT[lnwire.MilliSatoshi],
+	]
+
+	// FirstHopWireCustomRecords is a set of custom TLV records to include
+	// in the wire message sent to the first hop.
+	FirstHopWireCustomRecords lnwire.CustomRecords
+}
+
+// FinalHop returns the last hop in the route.
+func (r *Route) FinalHop() *Hop {
+	return r.Hops[len(r.Hops)-1]
+}
+
+// ReceiverAmt returns the amount forwarded to the final hop.
+func (r *Route) ReceiverAmt() lnwire.MilliSatoshi {
+	return r.FinalHop().AmtToForward
+}
+
+// TotalFees returns the total fees paid along the route.
+func (r *Route) TotalFees() lnwire.MilliSatoshi {
+	return r.TotalAmount - r.ReceiverAmt()
+}