Dual-funding can leave peers unable to reconnect

[rustyrussell]

This comes from a test flake in tests/test_connection.py::test_disconnect_opener.

l1's dualopend sends the final TX_COMPLETE, but l2 never gets it:

lightningd-d DEBUG   0266e...-lightningd: peer_disconnected
lightningd-2 INFO    0266e...-dualopend-chan#6: Peer connection lost
lightningd-2 INFO    0266e...-chan#6: Unsaved peer failed. Deleting channel.
lightningd-1 DEBUG   022d...-connectd: dev_disconnect: -WIRE_TX_COMPLETE (WIRE_TX_COMPLETE)
lightningd-1 DEBUG   022d...-lightningd: peer_disconnected
lightningd-1 DEBUG   022d...-dualopend-chan#7: peer_out WIRE_TX_COMPLETE
lightningd-1 DEBUG   022d...-hsmd: Got WIRE_HSMD_SETUP_CHANNEL
lightningd-1 DEBUG   hsmd: Client: Received message 31 from client
lightningd-1 DEBUG   022d...-hsmd: Got WIRE_HSMD_SIGN_REMOTE_COMMITMENT_TX
lightningd-1 DEBUG   hsmd: Client: Received message 19 from client
lightningd-2 DEBUG   hsmd: Client: Received message 34 from client
lightningd-2 DEBUG   plugin-funder: Cleaning up inflights for peer id 0266e...
lightningd-1 DEBUG   plugin-funder: Cleaning up inflights for peer id 022d...
lightningd-2 DEBUG   plugin-funder: Cleaning up inflight for channel_id fb2019cd9d0aea6efbf4b04ea7e45c28a7672f3b49c1897bde3dc99902b04cb6
lightningd-1 DEBUG   022d...-dualopend-chan#7: signature 3044022028d44889ad8c19bb414f15508641f8702d7b2763d25f191da0f29e0463ee2884022042c0b568f3fe0844521920377da3ff26e968485496b9c602b74aab151b17c2ed01 on tx 02000000014727a12e5631ea1bc9bc8409da7d21ca82998f205d8af46175fe2e7acca615b401000000008d320d80024a010000000000002200208e6510028e2e5646687cb835a6426185f041f97fdc1af9641bec49500b74b35c45b0000000000000220020d7742bbf56789f78631e99e0d90a08d3e039992c099c147b3d0eb090ca5eb24614a9f820 using key 02195f7b6a1012ea8948664019d7430fb4050329064a0ba6803ec55408ee9f10cb
lightningd-1 DEBUG   022d...-dualopend-chan#7: billboard: channel open: commitment ready to send, sending channel to lightningd to save

On every successive reconnect, l1 tries to REESTABLISH the channel, l2 hangs up:

lightningd-1 2026-01-07T04:44:37.367Z DEBUG   022d...-dualopend-chan#7: peer_out WIRE_CHANNEL_REESTABLISH
lightningd-1 2026-01-07T04:44:37.367Z DEBUG   022d...-dualopend-chan#7: billboard: Sent reestablish, waiting for theirs
lightningd-2 2026-01-07T04:44:37.377Z DEBUG   0266e...-connectd: Activating for message WIRE_CHANNEL_REESTABLISH
lightningd-2 2026-01-07T04:44:37.377Z UNUSUAL 0266e...-lightningd: Unknown channel fb2019cd9d0aea6efbf4b04ea7e45c28a7672f3b49c1897bde3dc99902b04cb6 for WIRE_CHANNEL_REESTABLISH
lightningd-2 2026-01-07T04:44:37.377Z DEBUG   0266e...-lightningd: Telling connectd to send error 0011fb2019cd9d0aea6efbf4b04ea7e45c28a7672f3b49c1897bde3dc99902b04cb6002c556e6b6e6f776e206368616e6e656c20666f7220574952455f4348414e4e454c5f524545535441424c495348

One fix would be not to hang up after sending an error for a channel. But a real fix would be nice!