feat(electrum): add blockchain.transaction.broadcast_package

[EddieHouston]

Summary

Adds the blockchain.transaction.broadcast_package Electrum method (part of Electrum protocol 1.6), letting Electrum clients broadcast a transaction package via bitcoind's submitpackage. This is a step toward #172, and directly enables the TRUC/v3 package-relay use case requested there (LDK, RewindBitcoin).

It reuses the existing submitpackage plumbing introduced in 5e36b93 (POST /txs/package): daemon.submit_package / query.submit_package / SubmitPackageResult.

Ported from @romanz's electrs (romanz#1242).

Behavior

Params: [[txhex, ...]] or [[txhex, ...], verbose].

• Non-verbose: returns { "success": bool }, or { "success": bool, "errors": [{ "error", "txid" }] } if any tx was rejected.
• Verbose: returns the full submitpackage result.

Local mempool visibility fix

query.submit_package previously only forwarded to bitcoind. Unlike query.broadcast_raw (which calls Mempool::add_by_txid), it did not add accepted txs to electrs' local mempool — so scripthash subscription/history updates (which read the local mempool) would miss package txs until the next background sync. This affected both the new Electrum method and the existing REST /txs/package endpoint.

Fixed at the shared query.submit_package layer: accepted txs are now added to the local mempool (new Mempool::add_by_txids, batched so parent/child package txs link within one add), benefiting both entry points.

Test plan

• test_electrum_broadcast_package — success + verbose response shapes.
• test_electrum_broadcast_package_updates_mempool — regression: a tx electrs hasn't synced becomes visible in scripthash history immediately after broadcast_package.
• test_rest_package_updates_mempool — same regression for POST /txs/package.

Both regression tests were verified to fail without the local-mempool fix. Full suite green on both default and --features liquid.

Ported from @romanz's electrs.