One transaction from getblocktemplate. Carries the full decoded
body and stats so block 0 can be projected without a follow-up
getmempoolentry/getrawtransaction per tx; that follow-up was the
source of the GBT/listing race that used to skip cycles.
Mempool snapshot data that survives one fetch cycle: the live
txid set, fee floor, and chain tip. Returned alongside the raw
block_template (which Fetcher consumes for GBT synthesis) by
Client::fetch_mempool_state.