lightningd: allow another gossip state transition.

This can definitely happen with zeroconf and the about-to-be-implemented withheld=True:

```
lightningd-1 2025-09-12T13:17:50.848Z **BROKEN** 022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59-chan#1: Illegal gossip state transition: CGOSSIP_WAITING_FOR_SCID->CGOSSIP_CHANNEL_UNANNOUNCED_DYING
```

Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

lightningd/channel_gossip.c

@@ -94,6 +94,8 @@ static struct state_transition allowed_transitions[] = {
 	  "Channel usable (zeroconf) but no scid yet" },
 	{ CGOSSIP_WAITING_FOR_SCID, CGOSSIP_CHANNEL_DEAD,
 	  "Zeroconf channel closed before funding tx mined" },
+	{ CGOSSIP_WAITING_FOR_SCID, CGOSSIP_CHANNEL_UNANNOUNCED_DYING,
+	  "Zeroconf channel closing mutually before funding tx" },
 	{ CGOSSIP_WAITING_FOR_USABLE, CGOSSIP_WAITING_FOR_MATCHING_PEER_SIGS,
 	  "Channel mined, but we haven't got matching announcment sigs from peer" },
 	{ CGOSSIP_WAITING_FOR_USABLE, CGOSSIP_WAITING_FOR_ANNOUNCE_DEPTH,