refactor: miscellaneous GenTxid followups

Addresses a few comments from #32631:
https://github.com/bitcoin/bitcoin/pull/32631#discussion_r2199951996
https://github.com/bitcoin/bitcoin/pull/32631#discussion_r2201874252
https://github.com/bitcoin/bitcoin/pull/32631#discussion_r2201918072

src/txmempool.cpp

@@ -962,9 +962,9 @@ const CTransaction* CTxMemPool::GetConflictTx(const COutPoint& prevout) const
 
 std::optional<CTxMemPool::txiter> CTxMemPool::GetIter(const Txid& txid) const
 {
+    AssertLockHeld(cs);
     auto it = mapTx.find(txid.ToUint256());
-    if (it != mapTx.end()) return it;
+    return it != mapTx.end() ? std::make_optional(it) : std::nullopt;
-    return std::nullopt;
 }
 
 std::optional<CTxMemPool::txiter> CTxMemPool::GetIter(const Wtxid& wtxid) const

src/txrequest.cpp

@@ -595,7 +595,7 @@ public:
 
     //! Find the GenTxids to request now from peer.
     std::vector<GenTxid> GetRequestable(NodeId peer, std::chrono::microseconds now,
-                                               std::vector<std::pair<NodeId, GenTxid>>* expired)
+                                        std::vector<std::pair<NodeId, GenTxid>>* expired)
     {
         // Move time.
         SetTimePoint(now, expired);
@@ -746,7 +746,7 @@ void TxRequestTracker::ReceivedResponse(NodeId peer, const uint256& txhash)
 }
 
 std::vector<GenTxid> TxRequestTracker::GetRequestable(NodeId peer, std::chrono::microseconds now,
-                                                             std::vector<std::pair<NodeId, GenTxid>>* expired)
+                                                      std::vector<std::pair<NodeId, GenTxid>>* expired)
 {
     return m_impl->GetRequestable(peer, now, expired);
 }

src/txrequest.h

@@ -164,7 +164,7 @@ public:
      *    simultaneously by one peer, and end up being requested from them, the requests will happen in announcement order.
      */
     std::vector<GenTxid> GetRequestable(NodeId peer, std::chrono::microseconds now,
-                                               std::vector<std::pair<NodeId, GenTxid>>* expired = nullptr);
+                                        std::vector<std::pair<NodeId, GenTxid>>* expired = nullptr);
 
     /** Marks a transaction as requested, with a specified expiry.
      *

src/util/transaction_identifier.h

@@ -98,7 +98,8 @@ public:
 
     friend auto operator<=>(const GenTxid& a, const GenTxid& b)
     {
-        return std::tuple(a.IsWtxid(), a.ToUint256()) <=> std::tuple(b.IsWtxid(), b.ToUint256());
+        // Use a reference for read-only access to the hash, avoiding a copy that might not be optimized away.
+        return std::tuple<bool, const uint256&>(a.IsWtxid(), a.ToUint256()) <=> std::tuple<bool, const uint256&>(b.IsWtxid(), b.ToUint256());
     }
 };