[rearden-grok]

Summary

• getblockheader returns a stored header that is no longer on the active chain (confirmations: -1, its own previousblockhash). Genesis omits previousblockhash. Active headers include nextblockhash and target. Fixes rpc: getblockheader returns -5 "Block not found" for a header that is no longer on the active chain, though getchaintips lists it #853.
• getblock rebuilds a Class A body for that same hash, and still serves a held never-confirmed body.
• getblockchaininfo adds bits and target. Difficulty uses 16 significant digits. validateaddress sets isscript for P2TR and P2A and omits witness fields on pay-to-anchor. getmempoolinfo adds fullrbf (true), maxdatacarriersize (null), and the live cluster limits. BTC amounts print as 8-decimal numbers, including scantxoutset total_amount. Fixes rpc: output differences from Bitcoin Core in getblockheader, getblock, getblockchaininfo, getdifficulty, validateaddress and getmempoolinfo #854.
• getblock verbosity 1 size / strippedsize / weight is already on master (rpc: getblock verbosity 1 carries size, strippedsize, and weight #847). This pull request does not redo it.

Test plan

• cargo test -p rbitcoin-rpc --lib -- --exact methods::chain::core_double_tests::sixteen_significant_digits
• cargo test -p rbitcoin-rpc --lib -- --exact methods::tests::getblock_named_verbose_genesis_and_hex
• cargo test -p rbitcoin-rpc --lib -- --exact methods::tests::getmempoolinfo_permitbaremultisig_is_always_true
• cargo test -p rbitcoin-rpc --lib -- --exact methods::tests::decode_rpc_subset
• cargo test -p rbitcoin-rpc --lib -- --exact methods::tests::rpc_regtest_chain_ops
• cargo clippy -p rbitcoin-rpc -p rbitcoin-net -p rbitcoin-mempool --all-targets -- -D warnings

[rearden-grok]

Review findings:

1. The new inactive-header paths set mediantime to the block's own timestamp in both off_chain_header_value and disconnected_block_value. Core reports the median time past over the preceding 11 headers on that branch, so this diverges for inactive chains with more than one header. Please calculate branch MTP from the stored parent links (or omit the field if it cannot be computed).

2. off_chain_header performs a parent-header lookup, an active-chain hash lookup, and then another parent-header lookup on each iteration. A long stored side branch can therefore turn a single getblockheader / getblock request into hundreds of thousands of random store lookups (the walk permits 100,000 steps). Please avoid the duplicate parent read and use a bounded/indexed ancestry walk so stale branches cannot make RPC requests scan the whole branch.