Drop archived bitcoincore-rpc dependency

[PeterXMR]

Closes #1254.

The upstream rust-bitcoincore-rpc crate was archived on 2025-11-25. This PR removes it from electrs in favor of a small in-tree rpc module that calls bitcoind directly via the still-maintained jsonrpc crate (which electrs was already pulling in transitively and using directly in batch_request).

Approach

I noticed bitcoincore-rpc was mostly a thin typed wrapper over jsonrpc::Client, and electrs uses only ~10 of its methods — so the replacement is narrow:

• Auth — local enum mirroring the three variants config.rs constructs.
• Client — wraps jsonrpc::Client and exposes the eight typed methods Daemon calls (get_network_info, get_blockchain_info, estimate_smart_fee, send_raw_transaction, get_raw_transaction, get_block_info, get_mempool_info, get_raw_mempool) plus the generic call() and get_jsonrpc_client() used for batch requests.
• Response types — #[derive(Deserialize)] structs containing only the fields electrs actually reads, matching the JSON-field renames bitcoind uses (networkactive, initialblockdownload, relayfee, feerate).
• Error — two-variant enum (JsonRpc | Decode) mirroring the nested shape callers already pattern-match against, so the changes in daemon.rs and electrum.rs are nearly mechanical.

Diff

Cargo.lock      |  38 +-------- (bitcoincore-rpc + -rpc-json removed; no new deps)
Cargo.toml      |   2 +-        (drop bitcoincore-rpc, promote jsonrpc to direct)
src/config.rs   |   2 +-        (import swap)
src/daemon.rs   |  19 +++-----
src/electrum.rs |   2 +-        (downcast_ref target)
src/lib.rs      |   1 +         (mod rpc;)
src/rpc.rs      | 201 +++++++++ (new)

Testing

• cargo build --all-targets — clean
• cargo clippy --all-targets -- -D warnings — zero warnings
• cargo test --lib — 21/21 pass (including config::tests::test_auth_debug, which exercises the local Auth enum's Debug formatting)
• Integration smoke test against Bitcoin Core 31 on regtest: drove every replaced RPC method end-to-end via the Electrum protocol:
  • blockchain.relayfee → get_network_info::relay_fee (BTC↔Amount serde)
  • blockchain.estimatefee → estimate_smart_fee (correctly maps missing feerate to Option<Amount>::None on regtest)
  • blockchain.transaction.get (hex) → get_raw_transaction (hex + consensus decode)
  • blockchain.transaction.get (verbose) → generic Client::call
  • mempool.get_fee_histogram → mempool sync exercising get_mempool_info + get_raw_mempool + batch getmempoolentry (incl. fees.base, vsize, depends) + batch getrawtransaction
  • blockchain.transaction.broadcast (fresh signed tx) → send_raw_transaction
  • error path: a deliberately-rejected rebroadcast returned through electrs as {"code": 2, ...}, confirming extract_bitcoind_error and the downcast_ref::<crate::rpc::Error>() in electrum.rs work as before.
• Implicitly exercised at startup: get_blockchain_info (rpc_poll), get_block_info (indexing 111 blocks), get_mempool_info.

Notes

• I left Auth::None in place (marked #[allow(dead_code)]) for API completeness — daemon.rs matches it but no current code path constructs it. Easy to remove if you'd prefer.
• I deliberately did not add an EstimateMode enum (the old bitcoincore_rpc_json::EstimateMode) since electrs always passes None for it. The new estimate_smart_fee takes Option<&str> instead, which avoids dead variants. Happy to add the enum back if you'd rather keep the API surface.
• I'm aware of Switch to bindex #1252 (bindex) — if that's expected to land soon and will rewrite most of daemon.rs anyway, this PR could be deferred or closed. It's a tidy interim cleanup either way.