One transaction from getblocktemplate: Core’s stats plus the still
encoded body, so block 0 can be projected without a follow-up
getmempoolentry/getrawtransaction per tx, and a poll decodes only the
bodies its caller doesn’t already hold.
How to reach a Bitcoin Core node: its data directory, block files and RPC endpoint.
Unset fields fall back to Bitcoin Core’s defaults; callers expand ~ themselves.