feat(bch): author shares at V36 from genesis + fail-loud V35 hash_link tripwire

src/impl/bch/pool_entrypoint.hpp

@@ -191,11 +191,11 @@ inline void standup_pool_run(boost::asio::io_context& ioc,
         -> core::stratum::RefHashResult
         {
             core::stratum::RefHashResult result;
-            result.share_version   = 35;
+            result.share_version   = 36;   // v36-native BCH sharechain (was 35, verify-preimage fix)
             result.desired_version = 36;
 
             bch::RefHashParams p;
-            p.share_version      = 35;
+            p.share_version      = 36;   // v36-native genesis default (was 35)
             p.prev_share         = prev_share_hash;
             p.coinbase_scriptSig = scriptSig;
             p.share_nonce        = 0;
@@ -505,7 +505,7 @@ inline void standup_pool_run(boost::asio::io_context& ioc,
             // reconstruction is byte-exact; fall back to the prev-tip version
             // (cold start: v35 voting v36) when ref_hash_fn produced no frozen
             // data (bits == 0).
-            int64_t create_ver = 35;
+            int64_t create_ver = 36;   // author V36 from genesis (was 35): empty-chain V35 hash_link cannot carry this coinbase
             if (job.frozen_ref.bits != 0) {
                 create_ver = job.frozen_ref.share_version;
             } else if (!job.prev_share_hash.IsNull() &&

src/impl/bch/share_check.hpp

@@ -170,6 +170,19 @@ inline HashLinkType prefix_to_hash_link_v35(
     const std::vector<unsigned char>& const_ending)
 {
     auto v36_link = prefix_to_hash_link(prefix, const_ending);
+    // FAIL-LOUD (v36 verify-preimage): a V35 HashLinkType (FixedStrType(0)) has
+    // NO extra_data field, so any coinbase whose trailing partial SHA-256 block
+    // exceeds the const_ending tail (bufsize > len(const_ending)) leaves residual
+    // bytes a V35 hash_link cannot carry. Silently dropping them makes
+    // check_hash_link recompute a DIFFERENT coinbase txid than the author
+    // committed, so the share fails verify against a v36 peer. This is exactly
+    // the case the oracle's V36 hash_link (variable-length extra_data) exists
+    // for: such a coinbase MUST be authored at V36. Refuse to forge an
+    // unrepresentable V35 hash_link rather than emit a mismatching preimage.
+    if (!v36_link.m_extra_data.m_data.empty())
+        throw std::runtime_error(
+            "prefix_to_hash_link_v35: coinbase requires V36 hash_link "
+            "(extra_data non-empty; V35 FixedStrType(0) cannot represent it)");
     HashLinkType result;
     result.m_state = v36_link.m_state;
     result.m_length = v36_link.m_length;