Fix merged mining stuck height: force aux work refresh after block submit

After submitting a merged block (submitblock or submitauxblock), the
poll_loop skipped createauxblock refresh because the daemon's best tip
hadn't changed yet (race with p2pool's submission at same height).

Result: c2pool kept mining against the same stale template, submitting
duplicate blocks at the same height repeatedly (all orphaned).

Fix: clear chain.last_tip after every block submission to force
refresh_aux_work() to call createauxblock on next poll cycle.

Author: frstrtr

src/c2pool/merged/merged_mining.cpp

@@ -784,6 +784,8 @@ void MergedMiningManager::try_submit_merged_blocks(
                              << payouts.size() << " payout outputs)";
                     bool ok = chain.rpc->submit_block(block_hex);
                     LOG_INFO << "[MM:" << chain.config.symbol << "] submit_block returned ok=" << ok;
+                    // Force aux work refresh on next poll — the tip may have changed
+                    chain.last_tip.clear();
                     try {
                         record_discovered_block(chain, ok, parent_hash.GetHex());
                         LOG_INFO << "[MM:" << chain.config.symbol << "] record_discovered_block OK";
@@ -827,6 +829,8 @@ void MergedMiningManager::submit_aux_and_relay(ChainState& chain, const std::str
                                                 const std::string& parent_hash_hex)
 {
     bool ok = chain.rpc->submit_aux_block(chain.current_work.block_hash, auxpow);
+    // Force aux work refresh — tip has changed
+    chain.last_tip.clear();
     record_discovered_block(chain, ok, parent_hash_hex);
     if (ok) {
         // Fetch the accepted block and relay via P2P for fast propagation