hsmd, splicing: wrong remote per-commit point provided when signing splice remote tx

### Issue and Steps to Reproduce

We (VLS) are seeing remote per-commitment point number 1 provided to signer instead of point number 0. This occurs when signing the remote commitment transaction on top of the new splice.

We have a `vlsd` log excerpt in https://gitlab.com/lightning-signer/vls-hsmd/-/merge_requests/132#note_1649538928 with the captured hsmd protocol messages.

I believe this happens when running any of the CLN `test_splicing.py` system tests.