dualopend: handle ANNOUNCEMENT_SIGNATURES from peer.

This can happen if we haven't transitioned to channeld yet, but logic is simply to hand
it to lightningd, exactly as channeld does.

```
2025-09-30T03:04:57.8951627Z lightningd-1 2025-09-30T02:59:14.150Z DEBUG   022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59-dualopend-chan#1: peer_out WIRE_WARNING
2025-09-30T03:04:57.8952126Z lightningd-1 2025-09-30T02:59:14.150Z **BROKEN** 022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59-dualopend-chan#1: Unexpected message WIRE_ANNOUNCEMENT_SIGNATURES
2025-09-30T03:04:57.8952521Z lightningd-1 2025-09-30T02:59:14.150Z INFO    022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59-dualopend-chan#1: Peer connection lost
2025-09-30T03:04:57.8953124Z lightningd-1 2025-09-30T02:59:14.150Z INFO    022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59-chan#1: Peer transient failure in DUALOPEND_AWAITING_LOCKIN: dualopend: Owning subdaemon dualopend died (62208)
```

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/dual_open_control.c

@@ -3696,6 +3696,28 @@ static void handle_commit_received(struct subd *dualopend,
 	abort();
 }
 
+static void handle_dualopend_got_announcement(struct subd *dualopend, const u8 *msg)
+{
+	struct channel *channel = dualopend->channel;
+	secp256k1_ecdsa_signature remote_ann_node_sig;
+	secp256k1_ecdsa_signature remote_ann_bitcoin_sig;
+	struct short_channel_id scid;
+
+	if (!fromwire_dualopend_got_announcement(msg,
+						 &scid,
+						 &remote_ann_node_sig,
+						 &remote_ann_bitcoin_sig)) {
+		channel_internal_error(channel,
+				       "bad dualopend_got_announcement %s",
+				       tal_hex(tmpctx, msg));
+		return;
+	}
+
+	channel_gossip_got_announcement_sigs(channel, scid,
+					     &remote_ann_node_sig,
+					     &remote_ann_bitcoin_sig);
+}
+
 static unsigned int dual_opend_msg(struct subd *dualopend,
 				   const u8 *msg, const int *fds)
 {
@@ -3758,6 +3780,9 @@ static unsigned int dual_opend_msg(struct subd *dualopend,
 		case WIRE_DUALOPEND_UPDATE_REQUIRE_CONFIRMED:
 			handle_update_require_confirmed(dualopend, msg);
 			return 0;
+		case WIRE_DUALOPEND_GOT_ANNOUNCEMENT:
+			handle_dualopend_got_announcement(dualopend, msg);
+			return 0;
 		/* Messages we send */
 		case WIRE_DUALOPEND_INIT:
 		case WIRE_DUALOPEND_REINIT:

openingd/dualopend.c

@@ -4169,12 +4169,33 @@ static u8 *handle_master_in(struct state *state)
 	case WIRE_DUALOPEND_VALIDATE_LEASE:
 	case WIRE_DUALOPEND_VALIDATE_INPUTS:
 	case WIRE_DUALOPEND_UPDATE_REQUIRE_CONFIRMED:
+	case WIRE_DUALOPEND_GOT_ANNOUNCEMENT:
 		break;
 	}
 	status_failed(STATUS_FAIL_MASTER_IO,
 		      "Unknown msg %s", tal_hex(tmpctx, msg));
 }
 
+static void handle_announcement_signatures(struct state *state, const u8 *msg)
+{
+	struct channel_id chanid;
+	struct short_channel_id remote_scid;
+	secp256k1_ecdsa_signature remote_node_sig, remote_bitcoin_sig;
+
+	if (!fromwire_announcement_signatures(msg,
+					      &chanid,
+					      &remote_scid,
+					      &remote_node_sig,
+					      &remote_bitcoin_sig))
+		open_err_fatal(state, "Bad announcement_signatures %s", tal_hex(msg, msg));
+
+	wire_sync_write(REQ_FD,
+			take(towire_dualopend_got_announcement(NULL,
+							       remote_scid,
+							       &remote_node_sig,
+							       &remote_bitcoin_sig)));
+}
+
 /*~ Standard "peer sent a message, handle it" demuxer.  Though it really only
  * handles a few messages, we use the standard form as principle of least
  * surprise. */
@@ -4221,6 +4242,9 @@ static u8 *handle_peer_in(struct state *state)
 	case WIRE_TX_ABORT:
 		handle_tx_abort(state, msg);
 		return NULL;
+	case WIRE_ANNOUNCEMENT_SIGNATURES:
+		handle_announcement_signatures(state, msg);
+		return NULL;
 		/* Otherwise we fall through */
 	case WIRE_INIT:
 	case WIRE_ERROR:
@@ -4239,7 +4263,6 @@ static u8 *handle_peer_in(struct state *state)
 	case WIRE_UPDATE_FEE:
 	case WIRE_UPDATE_BLOCKHEIGHT:
 	case WIRE_CHANNEL_REESTABLISH:
-	case WIRE_ANNOUNCEMENT_SIGNATURES:
 	case WIRE_GOSSIP_TIMESTAMP_FILTER:
 	case WIRE_ONION_MESSAGE:
 	case WIRE_ACCEPT_CHANNEL2:

openingd/dualopend_wire.csv

@@ -288,3 +288,8 @@ msgdata,dualopend_validate_lease,their_pubkey,pubkey,
 
 msgtype,dualopend_validate_lease_reply,7127
 msgdata,dualopend_validate_lease_reply,err_msg,?wirestring,
+
+msgtype,dualopend_got_announcement,7031
+msgdata,dualopend_got_announcement,scid,short_channel_id,
+msgdata,dualopend_got_announcement,remote_ann_node_sig,secp256k1_ecdsa_signature,
+msgdata,dualopend_got_announcement,remote_ann_bitcoin_sig,secp256k1_ecdsa_signature,