[LusoCryptoLabs]

merge_external_funding_witnesses is meant to fill our empty or placeholder witnesses from the peer's TxSignatures. is_empty_or_placeholder_witness passes witness.as_slice() to is_secp_sighash_placeholder_witness, which expects the raw witness. as_slice() of a packed::Bytes carries molecule's 4-byte length header, so the zero-signature WitnessArgs the funding builder writes for the peer's secp256k1 inputs is never taken for a placeholder. The initiator keeps the zeros instead of the peer's signature, and its copy of the funding tx fails with -302 TransactionFailedToVerify (secp256k1 lock, error -11). The acceptor's copy is complete and gets the channel open.

• is_empty_or_placeholder_witness checks witness.raw_data().

This is also what #1713 needs to help in practice: with secp256k1 inputs on the acceptor's side, the merged set it replies with still holds the placeholder until this is fixed.

Seen on our testnet LSP (a fork with #1623; this code is the same on develop): openings sent a copy of the funding tx that failed with -302 at the LSP's input, the same size as the copy that was committed. Reproduced on a dev chain through a proxy that keeps every send_transaction: before the change, the initiator's copy of each opening had a zero signature at the acceptor's input, in both signing orders; after it, every copy was fully signed. With the change on our testnet LSP, the next four openings (two from secp256k1 keys, two from passkey wallets, one of them in a UDT, in both signing orders) sent no copy that failed; with the binary before it, each of the five openings that day did.

Checked on this branch:

• test_external_funding_witness_merge_replaces_secp_placeholders: a local placeholder takes the peer's signature, and a peer's placeholder does not replace ours. It fails on develop.
• the other external funding tests
• cargo fmt, typos, cargo shear, make clippy
• cargo test -p fnn --features rocksdb: all pass except fiber::tests::mpp::test_mpp_waiting_ack_settlement_hands_off_to_durable_retry_queue, which fails on develop too