Genesis bootstrap: use max_target when sharechain is empty

When share_bits==0 (empty chain), miners couldn't create shares because
the bits guard blocked share creation and VARDIFF had no floor reference.

Fix: fall back to m_share_max_bits (or hardcoded 0x1e0fffff for LTC
testnet) when share_bits is zero. This matches p2pool's behavior of
setting desired_share_target = 2**256-1 when chain is empty, allowing
miners to create genesis shares immediately.

Also applies max_target as VARDIFF floor during bootstrap — prevents
miners from working at uselessly low difficulty when no share target
reference exists.

Author: frstrtr

src/core/stratum_server.cpp

@@ -198,6 +198,7 @@ void StratumSession::process_message(std::size_t bytes_read)
             // Floor initial difficulty at share target so miner doesn't waste
             // bandwidth submitting solutions that can never become real shares.
             uint32_t sb = mining_interface_->m_share_bits.load();
+            if (sb == 0) sb = mining_interface_->m_share_max_bits.load(); // bootstrap: use max target
             if (sb != 0) {
                 double share_diff = chain::target_to_difficulty(chain::bits_to_target(sb));
                 if (initial_diff < share_diff)

src/core/web_server.cpp

@@ -4442,6 +4442,14 @@ nlohmann::json MiningInterface::mining_submit(const std::string& username, const
                     params.prev_share_hash = job->prev_share_hash;
             }
 
+            // Bootstrap: when chain is empty (bits==0), use max_target
+            // so miners can create genesis shares immediately.
+            // P2Pool sets desired_share_target = 2**256-1 when chain is empty.
+            if (params.bits == 0) {
+                params.bits = m_share_max_bits.load();
+                if (params.bits == 0)
+                    params.bits = 0x1e0fffff; // fallback: easiest LTC testnet target
+            }
             if (!params.payout_script.empty() && params.bits != 0) {
                 m_create_share_fn(params);
             }