esplora: paged history, summary fields, and tx JSON

[rearden-grok]

Summary

Fixes the Esplora HTTP defects in #850, #852, #856, and #857.

• Paged /txs, /txs/chain, and /txs/summary are served when a script has more creates than --max-sh-creates. The full join (and the last-1 slot) is still built when the script is under the cap, so wallet reuse is unchanged. Unpaged stats and /utxo still return 503.
• /txs/summary rows include tx_position (the tx's index in its block). ?asof= is accepted on /txs/summary and /txs/summary/:last, which COMPAT.md already documented. The page size stays 25, matching COMPAT.md; mempool.space's unpaged full history is left unchanged.
• Tx JSON omits an empty witness. inner_redeemscript_asm is only the P2SH redeem script. inner_witnessscript_asm is a P2WSH witness script (including P2SH-P2WSH) or a Taproot script-path leaf (annex stripped). A key-path spend and an unknown prevout emit neither inner field.
• GET /block/:hash/txs/:start at or past the last tx returns 404 start index out of range. A start that is not a multiple of 25 stays 400.

Test plan

• cargo test -p rbitcoin-query --lib sh_history_caps
• cargo test -p rbitcoin-esplora --lib esplora_sh_join_and_template
• cargo test -p rbitcoin-esplora --lib esplora_tcp_wallet_mempool_and_replace
• cargo test -p rbitcoin-esplora --lib vin_inner_scripts_follow_prevout_type
• cargo test -p rbitcoin-esplora --lib block_txs_past_end_is_not_found
• cargo clippy -p rbitcoin-esplora -p rbitcoin-query --all-targets -- -D warnings

[rearden-grok]

Review finding:

The new tx_position lookup loads the complete transaction-FK list for each distinct block height in a summary page via block_tx_fks, then linearly searches it. A 25-row page can therefore add up to 25 random on-disk list reads and allocate/scan each whole block list, even though each row needs only one transaction's position. Please add a position lookup that addresses the needed transaction directly, or otherwise bound and measure this extra IO for summary requests.