Fix coinbase output extraction to support all outputs, not just witness commitment

The previous implementation only extracted the witness commitment output,
ignoring all other coinbase outputs (such as merge mining commitments).

This fix extracts ALL coinbase outputs from the template, enabling proper
support for merge mining and other multi-output scenarios.

Verified with end-to-end testing:
- Bitcoin Core: Creates 4 outputs (payout + 2 OP_RETURNs + witness)
- sv2-tp: Now extracts and serializes all 4 outputs
- SRI Pool: Receives all 4 outputs correctly

Fixes issue #18

Author: xyephy

src/sv2/messages.cpp

@@ -17,10 +17,11 @@ node::Sv2NewTemplateMsg::Sv2NewTemplateMsg(const CBlockHeader& header, const CTr
     // The coinbase nValue already contains the nFee + the Block Subsidy when built using CreateBlock().
     m_coinbase_tx_value_remaining = static_cast<uint64_t>(coinbase_tx->vout[0].nValue);
 
-    m_coinbase_tx_outputs_count = 0;
-    if (witness_commitment_index != NO_WITNESS_COMMITMENT) {
-        m_coinbase_tx_outputs_count = 1;
-        m_coinbase_tx_outputs = {coinbase_tx->vout[witness_commitment_index]};
+    // Extract ALL coinbase outputs to support merge mining and other use cases
+    m_coinbase_tx_outputs_count = coinbase_tx->vout.size();
+    m_coinbase_tx_outputs.clear();
+    for (const auto& output : coinbase_tx->vout) {
+        m_coinbase_tx_outputs.push_back(output);
     }
 
     m_coinbase_tx_locktime = coinbase_tx->nLockTime;