tx-submission: encode GenTxId as a bare hash on the wire (drop the era tag)

The node-to-node encoding of a hard-fork `GenTxId` prefixes an era index (`dispatchEncoder`/`encodeNS`) before the era's txid.
For a `GenTxId` that tag carries nothing the receiver uses: across every Cardano era the id is a 32-byte hash, and a tx's era is recovered from its body when fetched.
(Unlike `GenTx`, headers, and blocks, which have era-specific formats and do need the tag.)

A bare-hash txid on the wire would shrink tx-submission messages and let the consensus id become a plain hash, removing the `OneEraGenTxId`/`Maybe` bookkeeping that exists only to reconstruct the tag for the wire (see the product id on the `js/karknu/tx_undecision` branch).

This is a protocol change: new node-to-node version, tx-submission CDDL, and cross-implementation agreement.
It touches `ouroboros-network` and the CDDL spec, not just the encoder in consensus.

Filed separately from #2003, which fixes the `Eq`/`Ord` allocation in consensus without touching the wire.

Start by tracing the named dispatchEncoder and encodeNS entry points in consensus, then inspect the ouroboros-network tx-submission code and its CDDL specification. Review the tx_undecision product id for context. Done means agreement on the protocol change, a new node-to-node version, matching CDDL, and cross-implementation support for the bare hash.

Written by the indexing model from the issue text.