test: fold genesis RPC edges onto rpc_regtest_from_genesis

[rearden-grok]

Summary

Twelve unit tests repeated RPC reads and rejects on a private ctx_empty or a second regtest hub. rpc_regtest_from_genesis already opens one hub and calls dispatch. Those checks now run on that client:

• getblockcount 0 was already tip_count at the end of the genesis refusals.
• Genesis coinbase is not an ordinary tx (getrawtransaction and REST /tx) and is not a UTXO after the journey's first block (gettxout and REST /getutxos).
• estimatesmartfee / estimaterawfee empty-result shape, conf-target gates, and the CLN target loop.
• getmempoolinfo constant fields, including a min-relay bump that is restored to the value the hub already had.
• scantxoutset matches an unknown method. gettxoutsetinfo, combinerawtransaction, and syncwithvalidationinterfacequeue stay unlisted.
• decoderawtransaction keeps a segwit coinbase witness.
• The echo named/positional matrix extends the echo beat this journey already had.
• getblockstats error needles use the headers-only block this journey already submits.

No second chain, server, or pad. Test-only. No changelog.

Test plan

• cargo fmt -p rbitcoin-rpc
• cargo clippy -p rbitcoin-rpc --tests -- -D warnings
• cargo test -p rbitcoin-rpc rpc_regtest_from_genesis
• GitHub qc, test, windows, macos, coverage