refactor: allow wallet to select fresh change address to prevent address reuse in coinjoin createdenoms

doc/release-notes-6870.md

@@ -0,0 +1,9 @@
+Wallet
+------
+
+- CoinJoin denomination creation now respects the wallet's "avoid_reuse"
+  setting. When the wallet has `avoid_reuse` enabled, change is sent to a
+  fresh change address to avoid address/public key reuse. Otherwise, change
+  goes back to the source address (legacy behavior). (#6870)
+
+

src/coinjoin/util.cpp

@@ -9,6 +9,7 @@
 #include <wallet/fees.h>
 #include <wallet/spend.h>
 #include <wallet/wallet.h>
+#include <wallet/walletutil.h>
 
 #include <numeric>
 
@@ -125,8 +126,13 @@ CTransactionBuilder::CTransactionBuilder(CWallet& wallet, const CompactTallyItem
     coinControl.m_discard_feerate = ::GetDiscardRate(m_wallet);
     // Generate a feerate which will be used by calculations of this class and also by CWallet::CreateTransaction
     coinControl.m_feerate = std::max(GetRequiredFeeRate(m_wallet), m_wallet.m_pay_tx_fee);
-    // Change always goes back to origin
-    coinControl.destChange = tallyItemIn.txdest;
+    // If wallet does not have the avoid-reuse feature enabled, keep legacy
+    // behavior: force change to go back to the origin address. When
+    // WALLET_FLAG_AVOID_REUSE is enabled, let the wallet select a fresh
+    // change destination to avoid address reuse.
+    if (!m_wallet.IsWalletFlagSet(wallet::WALLET_FLAG_AVOID_REUSE)) {
+        coinControl.destChange = tallyItemIn.txdest;
+    }
     // Only allow tallyItems inputs for tx creation
     coinControl.m_allow_other_inputs = false;
     // Create dummy tx to calculate the exact required fees upfront for accurate amount and fee calculations