Channel stuck in CLOSINGD_COMPLETE if funding never confirms

When a mutual close negotiation completes on a channel whose funding
tx never confirmed, the channel state is stuck. The signed close tx
is permanently invalid — its input is a 2-of-2 funding output that
does not exist on chain — so the state machine has no path from
`CLOSINGD_COMPLETE` to `FUNDING_SPEND_SEEN` / `ONCHAIN`. The channel
sits indefinitely with no cleanup.

Similar to #9112

## Concrete instances

More details about the following channels can be found in [clboss
issue #312](https://github.com/ksedgwic/clboss/issues/312). @ksedgwic
operates one node; the issue reporter operates the other.

### Channel A

- `channel_id`: `98f1632d48f0d94d86161a8b4745a97d34c72beb3a6f1db4050210d18aae1e6e`
- `funding_txid`: `6f1eae8ad1100205b41d6f3aeb2bc7347da945478b1a16864dd9f0482d63f198` — never confirmed on chain
- close tx (signed, in DB on both sides): `92ce9629985b686b1b9b433b78561fe33d188c71c1131d100798bb24354fe57d` — never broadcast; 404 on every explorer
- opener: counterparty; closer: us
- created 2026-01-31; reached `CLOSINGD_COMPLETE` 2026-02-24
- still in `CLOSINGD_COMPLETE` on both sides as of 2026-04-30

### Channel B

- `channel_id`: `264d24bdade7908c31e4d8338368d321c08f4b50fbaf92761da239302ac2cf45`
- `funding_txid`: `45cfc22a3039a21d7692affb504b8fc021d3688333d8e4318c90e7adbd244d26` — never confirmed
- close tx (signed, in DB on both sides): `d00300b5acb9b7105d2e11d176a1bee5cf05db8e38b6edc6cd8224ff226665f8` — never broadcast
- opener: counterparty; closer: us
- created 2026-02-28; reached `CLOSINGD_COMPLETE` 2026-03-11
- still in `CLOSINGD_COMPLETE` on both sides as of 2026-04-30

## Reproducer

Reproduced by #9103