fix(rpc): verify fetched mempool tx txids

fix(rpc): verify fetched mempool tx txids

- Add `tx.compute_txid() == txid` check to validate that fetched raw
mempool transaction hashes against their requested txid before caching
in Emitter::mempool_at. Returns `Error::UnexpectedStructure` for a
mismatch.

- Add a regression test for the above check.

- Document that the crate assumes a trusted `bitcoind` connection, and
that the txid check does not cover witness data.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>