btc: pull peer mempool on connect (BIP 35) so embedded TemplateBuilder produces POPULATED blocks

The new_tx -> mempool.add_tx subscription in main_btc only sees txs
announced via inv AFTER connect. Txs already resident in the bitcoind
mempool at connect time (e.g. a seeded regtest mempool) were never
requested, so won blocks were coinbase-only (nTx=1) — the G3b POPULATED
residual.

Expose Node::enable_mempool_request() (forwards to NodeP2P, applied at
start_p2p time and to any later reconnect) and call it from main_btc
after start_p2p. Mirrors the established main_dgb pattern. Peer must
advertise NODE_BLOOM (regtest: -peerbloomfilters=1) or NodeP2P skips the
request (logged) to avoid a disconnect; normal inv relay is unchanged.

BTC-fenced; no cross-coin contract change.

Author: frstrtr

src/c2pool/main_btc.cpp

@@ -533,6 +533,15 @@ int main(int argc, char* argv[])
     LOG_INFO << "[BTC] Connecting to bitcoind...";
     coin_node.start_p2p(NetService(bitcoind_host, bitcoind_port));
 
+    // Phase 10c: pull the peer mempool on connect (BIP 35) so TemplateBuilder
+    // produces POPULATED blocks. Without this the new_tx subscription above only
+    // sees txs announced via inv AFTER connect; txs already resident in the
+    // bitcoind mempool at connect time (e.g. a seeded regtest mempool) are never
+    // requested, leaving coinbase-only templates. Mirrors main_dgb. Peer must
+    // advertise NODE_BLOOM (regtest: -peerbloomfilters=1) or the request is
+    // skipped (logged) to avoid a disconnect; normal inv relay still applies.
+    coin_node.enable_mempool_request();
+
     // Drive initial header sync. Per BTC protocol, NodeP2P's verack handler
     // sends sendheaders/sendcmpct/feefilter but NOT getheaders — header sync
     // is the consumer's responsibility (LTC drives this from the broadcaster).

src/impl/btc/coin/node.hpp

@@ -29,6 +29,7 @@ class Node : public btc::interfaces::Node
 
     std::unique_ptr<NodeRPC> m_rpc;
     std::unique_ptr<NodeP2P<config_t>> m_p2p;
+    bool m_request_mempool_on_connect{false};  // BIP 35 pull on (re)connect
 
     void init_p2p()
     {
@@ -79,10 +80,26 @@ class Node : public btc::interfaces::Node
     void start_p2p(const NetService& addr)
     {
         m_p2p = std::make_unique<NodeP2P<config_t>>(m_context, this, m_config);
+        if (m_request_mempool_on_connect)
+            m_p2p->enable_mempool_request();
         m_p2p->connect(addr);
         LOG_INFO << "Coin P2P broadcaster connecting to " << addr.to_string();
     }
 
+    /// Opt into the BIP 35 `mempool` request on (re)connect. Without this the
+    /// embedded mempool only learns txs announced via `inv` AFTER we connect,
+    /// so any tx already resident in the peer mempool at connect time is never
+    /// pulled -> coinbase-only templates. Mirrors main_dgb. Idempotent; applies
+    /// to the current peer immediately and to any peer start_p2p creates later.
+    /// NOTE: the peer must advertise NODE_BLOOM (regtest: -peerbloomfilters=1)
+    /// or NodeP2P skips the request to avoid a disconnect.
+    void enable_mempool_request()
+    {
+        m_request_mempool_on_connect = true;
+        if (m_p2p)
+            m_p2p->enable_mempool_request();
+    }
+
     /// Submit a block via P2P directly (faster propagation than RPC).
     void submit_block_p2p(BlockType& block)
     {