store: schema 25 txstat.body 8 B/create ULEB

[rearden-grok]

Summary

Schema 25 dense txstat.body (8 B/create canonical ULEB) stacked on #654 (store/inwit-prune). Occupied 24 zero-extends the file; txout.body is not rewritten. Unreleased leftover txfixed.body is unlinked.

• Confirm stamps n_in / fee_sat / base / wit_extra. Size and weight are derived (size = base + wit_extra, weight = 4×base + wit_extra). No sigops field. Overflow remaining-bytes live in per-header txstat.ovf + txstat.blk (not a global fk map).
• New LAYOUT17 writes set bit 4 and omit the n_in uleb; leftover 24 rows still decode. Overlay reads n_in from the first ULEB even when the cell needs a tail.
• getblockstats reads stamped rows (no inwit reconstruct). JSON keeps utxo_increase as outs − ins and omits Core coins-DB utxo_size_* / utxo_increase_actual.
• Pruned Esplora /tx keeps empty vin; stamped txstat fills fee / size / weight (no sigops). Named disk ~11.3 GiB, not process-resident.

Test plan

• cargo test -p rbitcoin-store --lib txstat
• cargo test -p rbitcoin-store --lib open_schema24_occupied_creates_zero_txstat
• cargo test -p rbitcoin-store --lib -- --skip sp_tweaks --skip pool_rmw
• cargo test -p rbitcoin-rpc --lib getblockstats_uses_txstat
• cargo test -p rbitcoin-rpc --lib getblockstats_reconstructs_when_txstat_unstamped
• cargo test -p rbitcoin-esplora --lib pruned_tx_json_has_fee_from_txstat
• cargo test -p rbitcoin-consensus --lib confirm_write_stamps_txstat
• cargo test -p rbitcoin-query --lib stamp_txstat_from_block
• cargo test -p rbitcoin-query --lib txstat_row_merges_overflow
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• ./scripts/ast-grep.sh
• required CI

Do not merge unless asked.