policy, refactor: Convert uint256 to Txid

diff --git a/src/policy/fees.cpp b/src/policy/fees.cpp
index e385f548..a650a132 100644
--- a/src/policy/fees.cpp
+++ b/src/policy/fees.cpp
@@ -519,16 +519,16 @@ void TxConfirmStats::removeTx(unsigned int entryHeight, unsigned int nBestSeenHe
     }
 }
 
-bool CBlockPolicyEstimator::removeTx(uint256 hash)
+bool CBlockPolicyEstimator::removeTx(Txid hash)
 {
     LOCK(m_cs_fee_estimator);
     return _removeTx(hash, /*inBlock=*/false);
 }
 
-bool CBlockPolicyEstimator::_removeTx(const uint256& hash, bool inBlock)
+bool CBlockPolicyEstimator::_removeTx(const Txid& hash, bool inBlock)
 {
     AssertLockHeld(m_cs_fee_estimator);
-    std::map<uint256, TxStatsInfo>::iterator pos = mapMemPoolTxs.find(hash);
+    std::map<Txid, TxStatsInfo>::iterator pos = mapMemPoolTxs.find(hash);
     if (pos != mapMemPoolTxs.end()) {
         feeStats->removeTx(pos->second.blockHeight, nBestSeenHeight, pos->second.bucketIndex, inBlock);
         shortStats->removeTx(pos->second.blockHeight, nBestSeenHeight, pos->second.bucketIndex, inBlock);
diff --git a/src/policy/fees.h b/src/policy/fees.h
index a95cc19d..b355b65a 100644
--- a/src/policy/fees.h
+++ b/src/policy/fees.h
@@ -212,7 +212,7 @@ public:
         EXCLUSIVE_LOCKS_REQUIRED(!m_cs_fee_estimator);
 
     /** Remove a transaction from the mempool tracking stats for non BLOCK removal reasons*/
-    bool removeTx(uint256 hash)
+    bool removeTx(Txid hash)
         EXCLUSIVE_LOCKS_REQUIRED(!m_cs_fee_estimator);
 
     /** DEPRECATED. Return a feerate estimate */
@@ -287,7 +287,7 @@ private:
     };
 
     // map of txids to information about that transaction
-    std::map<uint256, TxStatsInfo> mapMemPoolTxs GUARDED_BY(m_cs_fee_estimator);
+    std::map<Txid, TxStatsInfo> mapMemPoolTxs GUARDED_BY(m_cs_fee_estimator);
 
     /** Classes to track historical data on transaction confirmations */
     std::unique_ptr<TxConfirmStats> feeStats PT_GUARDED_BY(m_cs_fee_estimator);
@@ -315,7 +315,7 @@ private:
     unsigned int MaxUsableEstimate() const EXCLUSIVE_LOCKS_REQUIRED(m_cs_fee_estimator);
 
     /** A non-thread-safe helper for the removeTx function */
-    bool _removeTx(const uint256& hash, bool inBlock)
+    bool _removeTx(const Txid& hash, bool inBlock)
         EXCLUSIVE_LOCKS_REQUIRED(m_cs_fee_estimator);
 };
 
diff --git a/src/policy/packages.cpp b/src/policy/packages.cpp
index 693adcdf..187bd467 100644
--- a/src/policy/packages.cpp
+++ b/src/policy/packages.cpp
@@ -16,7 +16,7 @@
 
 /** IsTopoSortedPackage where a set of txids has been pre-populated. The set is assumed to be correct and
  * is mutated within this function (even if return value is false). */
-bool IsTopoSortedPackage(const Package& txns, std::unordered_set<uint256, SaltedTxidHasher>& later_txids)
+bool IsTopoSortedPackage(const Package& txns, std::unordered_set<Txid, SaltedTxidHasher>& later_txids)
 {
     // Avoid misusing this function: later_txids should contain the txids of txns.
     Assume(txns.size() == later_txids.size());
@@ -42,7 +42,7 @@ bool IsTopoSortedPackage(const Package& txns, std::unordered_set<uint256, Salted
 
 bool IsTopoSortedPackage(const Package& txns)
 {
-    std::unordered_set<uint256, SaltedTxidHasher> later_txids;
+    std::unordered_set<Txid, SaltedTxidHasher> later_txids;
     std::transform(txns.cbegin(), txns.cend(), std::inserter(later_txids, later_txids.end()),
                    [](const auto& tx) { return tx->GetHash(); });
 
@@ -91,7 +91,7 @@ bool IsWellFormedPackage(const Package& txns, PackageValidationState& state, boo
         return state.Invalid(PackageValidationResult::PCKG_POLICY, "package-too-large");
     }
 
-    std::unordered_set<uint256, SaltedTxidHasher> later_txids;
+    std::unordered_set<Txid, SaltedTxidHasher> later_txids;
     std::transform(txns.cbegin(), txns.cend(), std::inserter(later_txids, later_txids.end()),
                    [](const auto& tx) { return tx->GetHash(); });
 
@@ -123,7 +123,7 @@ bool IsChildWithParents(const Package& package)
 
     // The package is expected to be sorted, so the last transaction is the child.
     const auto& child = package.back();
-    std::unordered_set<uint256, SaltedTxidHasher> input_txids;
+    std::unordered_set<Txid, SaltedTxidHasher> input_txids;
     std::transform(child->vin.cbegin(), child->vin.cend(),
                    std::inserter(input_txids, input_txids.end()),
                    [](const auto& input) { return input.prevout.hash; });
@@ -136,7 +136,7 @@ bool IsChildWithParents(const Package& package)
 bool IsChildWithParentsTree(const Package& package)
 {
     if (!IsChildWithParents(package)) return false;
-    std::unordered_set<uint256, SaltedTxidHasher> parent_txids;
+    std::unordered_set<Txid, SaltedTxidHasher> parent_txids;
     std::transform(package.cbegin(), package.cend() - 1, std::inserter(parent_txids, parent_txids.end()),
                    [](const auto& ptx) { return ptx->GetHash(); });
     // Each parent must not have an input who is one of the other parents.
diff --git a/src/policy/rbf.cpp b/src/policy/rbf.cpp
index dc74f43b..210b9491 100644
--- a/src/policy/rbf.cpp
+++ b/src/policy/rbf.cpp
@@ -62,7 +62,6 @@ std::optional<std::string> GetEntriesForConflicts(const CTransaction& tx,
                                                   CTxMemPool::setEntries& all_conflicts)
 {
     AssertLockHeld(pool.cs);
-    const uint256 txid = tx.GetHash();
     uint64_t nConflictingCount = 0;
     for (const auto& mi : iters_conflicting) {
         nConflictingCount += mi->GetCountWithDescendants();
@@ -72,7 +71,7 @@ std::optional<std::string> GetEntriesForConflicts(const CTransaction& tx,
         // times), but we just want to be conservative to avoid doing too much work.
         if (nConflictingCount > MAX_REPLACEMENT_CANDIDATES) {
             return strprintf("rejecting replacement %s; too many potential replacements (%d > %d)",
-                             txid.ToString(),
+                             tx.GetHash().ToString(),
                              nConflictingCount,
                              MAX_REPLACEMENT_CANDIDATES);
         }
@@ -89,7 +88,7 @@ std::optional<std::string> HasNoNewUnconfirmed(const CTransaction& tx,
                                                const CTxMemPool::setEntries& iters_conflicting)
 {
     AssertLockHeld(pool.cs);
-    std::set<uint256> parents_of_conflicts;
+    std::set<Txid> parents_of_conflicts;
     for (const auto& mi : iters_conflicting) {
         for (const CTxIn& txin : mi->GetTx().vin) {
             parents_of_conflicts.insert(txin.prevout.hash);
@@ -118,7 +117,7 @@ std::optional<std::string> HasNoNewUnconfirmed(const CTransaction& tx,
 
 std::optional<std::string> EntriesAndTxidsDisjoint(const CTxMemPool::setEntries& ancestors,
                                                    const std::set<Txid>& direct_conflicts,
-                                                   const uint256& txid)
+                                                   const Txid& txid)
 {
     for (CTxMemPool::txiter ancestorIt : ancestors) {
         const Txid& hashAncestor = ancestorIt->GetTx().GetHash();
@@ -133,7 +132,7 @@ std::optional<std::string> EntriesAndTxidsDisjoint(const CTxMemPool::setEntries&
 
 std::optional<std::string> PaysMoreThanConflicts(const CTxMemPool::setEntries& iters_conflicting,
                                                  CFeeRate replacement_feerate,
-                                                 const uint256& txid)
+                                                 const Txid& txid)
 {
     for (const auto& mi : iters_conflicting) {
         // Don't allow the replacement to reduce the feerate of the mempool.
@@ -161,7 +160,7 @@ std::optional<std::string> PaysForRBF(CAmount original_fees,
                                       CAmount replacement_fees,
                                       size_t replacement_vsize,
                                       CFeeRate relay_fee,
-                                      const uint256& txid)
+                                      const Txid& txid)
 {
     // Rule #3: The replacement fees must be greater than or equal to fees of the
     // transactions it replaces, otherwise the bandwidth used by those conflicting transactions
diff --git a/src/policy/rbf.h b/src/policy/rbf.h
index 3cc0fc31..cc397b46 100644
--- a/src/policy/rbf.h
+++ b/src/policy/rbf.h
@@ -90,7 +90,7 @@ std::optional<std::string> HasNoNewUnconfirmed(const CTransaction& tx, const CTx
  */
 std::optional<std::string> EntriesAndTxidsDisjoint(const CTxMemPool::setEntries& ancestors,
                                                    const std::set<Txid>& direct_conflicts,
-                                                   const uint256& txid);
+                                                   const Txid& txid);
 
 /** Check that the feerate of the replacement transaction(s) is higher than the feerate of each
  * of the transactions in iters_conflicting.
@@ -98,7 +98,7 @@ std::optional<std::string> EntriesAndTxidsDisjoint(const CTxMemPool::setEntries&
  * @returns error message if fees insufficient, otherwise std::nullopt.
  */
 std::optional<std::string> PaysMoreThanConflicts(const CTxMemPool::setEntries& iters_conflicting,
-                                                 CFeeRate replacement_feerate, const uint256& txid);
+                                                 CFeeRate replacement_feerate, const Txid& txid);
 
 /** The replacement transaction must pay more fees than the original transactions. The additional
  * fees must pay for the replacement's bandwidth at or above the incremental relay feerate.
@@ -113,7 +113,7 @@ std::optional<std::string> PaysForRBF(CAmount original_fees,
                                       CAmount replacement_fees,
                                       size_t replacement_vsize,
                                       CFeeRate relay_fee,
-                                      const uint256& txid);
+                                      const Txid& txid);
 
 /**
  * The replacement transaction must improve the feerate diagram of the mempool.
diff --git a/src/test/fuzz/policy_estimator.cpp b/src/test/fuzz/policy_estimator.cpp
index 37e37964..8455a232 100644
--- a/src/test/fuzz/policy_estimator.cpp
+++ b/src/test/fuzz/policy_estimator.cpp
@@ -85,7 +85,7 @@ FUZZ_TARGET(policy_estimator, .init = initialize_policy_estimator)
                 block_policy_estimator.processBlock(txs, current_height);
             },
             [&] {
-                (void)block_policy_estimator.removeTx(ConsumeUInt256(fuzzed_data_provider));
+                (void)block_policy_estimator.removeTx(Txid::FromUint256(ConsumeUInt256(fuzzed_data_provider)));
             },
             [&] {
                 block_policy_estimator.FlushUnconfirmed();
diff --git a/src/test/rbf_tests.cpp b/src/test/rbf_tests.cpp
index 0052276e..cbbea61a 100644
--- a/src/test/rbf_tests.cpp
+++ b/src/test/rbf_tests.cpp
@@ -196,7 +196,7 @@ BOOST_FIXTURE_TEST_CASE(rbf_helper_functions, TestChain100Setup)
                                        entry5_low, entry6_low_prioritised, entry7_high, entry8_high};
     CTxMemPool::setEntries empty_set;
 
-    const auto unused_txid{GetRandHash()};
+    const auto unused_txid = Txid::FromUint256(GetRandHash());
 
     // Tests for PaysMoreThanConflicts
     // These tests use feerate, not absolute fee.
diff --git a/src/validation.cpp b/src/validation.cpp
index 69cc84bb..0d68475a 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -1088,7 +1088,7 @@ bool MemPoolAccept::ReplacementChecks(Workspace& ws)
     AssertLockHeld(m_pool.cs);
 
     const CTransaction& tx = *ws.m_ptx;
-    const uint256& hash = ws.m_hash;
+    const Txid& hash = ws.m_hash;
     TxValidationState& state = ws.m_state;
 
     CFeeRate newFeeRate(ws.m_modified_fees, ws.m_vsize);