BIP339 formalizes wtxid-based transaction relay

BIP 339 specified a new `wtxidrelay` negotiation message and inventory type so peers supporting the feature could identify transactions by the hash that includes witness data. Before this, transaction announcements continued to use txids that excluded the witness.

Using wtxids closes an inefficiency created by witness malleability: a node that rejects one witness version no longer needs to redownload another transaction with the same txid simply because the witness differs. The feature was backward compatible because it is negotiated only between peers that support it.

The change was infrastructure work rather than a consensus soft fork, but it improved relay behavior and also supported later deployment work around Taproot.