Set anti-fee-sniping locktime (exact-tip) on bitcoin sends (#3385)

diff --git a/cw_bitcoin/lib/bitcoin_wallet.dart b/cw_bitcoin/lib/bitcoin_wallet.dart
index b8e53d1..4f791ae 100644
--- a/cw_bitcoin/lib/bitcoin_wallet.dart
+++ b/cw_bitcoin/lib/bitcoin_wallet.dart
@@ -14,6 +14,7 @@ import 'package:cw_bitcoin/electrum_derivations.dart';
 import 'package:cw_bitcoin/electrum_transaction_info.dart';
 import 'package:cw_bitcoin/electrum_wallet.dart';
 import 'package:cw_bitcoin/electrum_wallet_snapshot.dart';
+import 'package:cw_bitcoin/locktime.dart';
 import 'package:cw_bitcoin/hardware/bitcoin_hardware_wallet_service.dart';
 import 'package:cw_bitcoin/lightning/lightning_wallet.dart';
 import 'package:cw_bitcoin/hardware/bitcoin_ledger_service.dart';
@@ -31,6 +32,7 @@ import 'package:cw_core/encryption_file_utils.dart';
 import 'package:cw_core/output_info.dart';
 import 'package:cw_core/payjoin_session.dart';
 import 'package:cw_core/pending_transaction.dart';
+import 'package:cw_core/sync_status.dart';
 import 'package:cw_core/unspent_coin_type.dart';
 import 'package:cw_core/unspent_coins_info.dart';
 import 'package:cw_core/utils/print_verbose.dart';
@@ -447,8 +449,17 @@ abstract class BitcoinWalletBase extends ElectrumWallet with Store {
       ));
     }
 
+    final locktime = antiFeeSnipingLocktime(
+      chainTip: await getCurrentChainTip(),
+      synced: syncStatus is SyncedSyncStatus,
+    );
+
     return PSBTTransactionBuild(
-            inputs: psbtReadyInputs, outputs: outputs, enableRBF: enableRBF, cwOutputs: cwOutputs)
+            inputs: psbtReadyInputs,
+            outputs: outputs,
+            enableRBF: enableRBF,
+            cwOutputs: cwOutputs,
+            locktime: locktime)
         .psbt;
   }
 
diff --git a/cw_bitcoin/lib/electrum_wallet.dart b/cw_bitcoin/lib/electrum_wallet.dart
index 857317a..b69fec5 100644
--- a/cw_bitcoin/lib/electrum_wallet.dart
+++ b/cw_bitcoin/lib/electrum_wallet.dart
@@ -4,6 +4,7 @@ import 'dart:isolate';
 
 import 'package:bitcoin_base/bitcoin_base.dart';
 import 'package:cw_bitcoin/lightning/lightning_wallet.dart';
+import 'package:cw_bitcoin/locktime.dart';
 import 'package:cw_core/hardware/hardware_wallet_service.dart';
 import 'package:cw_core/root_dir.dart';
 import 'package:cw_core/utils/proxy_wrapper.dart';
@@ -448,6 +449,15 @@ abstract class ElectrumWalletBase
     return currentChainTip ?? 0;
   }
 
+  /// Anti-fee-sniping locktime (current tip, exact-tip), LE-encoded for
+  /// `BitcoinTransactionBuilder`.
+  Future<List<int>> _antiFeeSnipingLocktime() async {
+    return locktimeToBytes(antiFeeSnipingLocktime(
+      chainTip: await getCurrentChainTip(),
+      synced: syncStatus is SyncedSyncStatus,
+    ));
+  }
+
   @override
   BitcoinWalletKeys get keys {
     String? wif;
@@ -1516,6 +1526,8 @@ abstract class ElectrumWalletBase
           });
       }
 
+      final locktime = await _antiFeeSnipingLocktime();
+
       BasedBitcoinTransacationBuilder txb;
       if (network is BitcoinCashNetwork) {
         txb = ForkedTransactionBuilder(
@@ -1540,6 +1552,7 @@ abstract class ElectrumWalletBase
           inputOrdering: BitcoinOrdering.shuffle,
           outputOrdering: BitcoinOrdering.shuffle,
           enableRBF: !estimatedTx.spendsUnconfirmedTX,
+          locktime: locktime,
         );
       }
 
@@ -2306,6 +2319,7 @@ abstract class ElectrumWalletBase
         inputOrdering: BitcoinOrdering.shuffle,
         outputOrdering: BitcoinOrdering.shuffle,
         enableRBF: true,
+        locktime: await _antiFeeSnipingLocktime(),
       );
 
       final transaction = txb.buildTransaction((txDigest, utxo, publicKey, sighash) {
diff --git a/cw_bitcoin/lib/locktime.dart b/cw_bitcoin/lib/locktime.dart
new file mode 100644
index 0000000..639e6e1
--- /dev/null
+++ b/cw_bitcoin/lib/locktime.dart
@@ -0,0 +1,20 @@
+import "dart:typed_data";
+
+/// Anti-fee-sniping locktime, set to the current chain tip (exact-tip).
+///
+/// Matches the exact-tip cluster (payjoin-cli, ldk-node, Bull Bitcoin) and Core's
+/// default for externally-constructed transactions, and takes Cake off its unique
+/// `nLockTime = 0` fingerprint. It intentionally skips the ~10% backdate
+/// Core/Electrum apply for delayed-broadcast privacy, which does not apply here.
+/// Returns 0 when not synced or the tip is unknown, to avoid emitting a stale
+/// height.
+int antiFeeSnipingLocktime({
+  required int chainTip,
+  required bool synced,
+}) {
+  if (!synced || chainTip <= 0) return 0;
+  return chainTip;
+}
+
+/// Little-endian 4-byte encoding for `BtcTransaction.locktime`.
+List<int> locktimeToBytes(int locktime) => (ByteData(4)..setUint32(0, locktime, Endian.little)).buffer.asUint8List();
diff --git a/cw_bitcoin/lib/psbt/transaction_builder.dart b/cw_bitcoin/lib/psbt/transaction_builder.dart
index 7d2ead1..ab107bf 100644
--- a/cw_bitcoin/lib/psbt/transaction_builder.dart
+++ b/cw_bitcoin/lib/psbt/transaction_builder.dart
@@ -14,8 +14,10 @@ class PSBTTransactionBuild {
       {required List<PSBTReadyUtxoWithAddress> inputs,
       required List<BitcoinBaseOutput> outputs,
       required List<OutputInfo> cwOutputs,
-      bool enableRBF = true}) {
+      bool enableRBF = true,
+      int locktime = 0}) {
     psbt.setGlobalTxVersion(2);
+    psbt.setGlobalFallbackLocktime(locktime);
     psbt.setGlobalInputCount(inputs.length);
     psbt.setGlobalOutputCount(outputs.length);
 
diff --git a/cw_bitcoin/test/locktime_test.dart b/cw_bitcoin/test/locktime_test.dart
new file mode 100644
index 0000000..cd20e02
--- /dev/null
+++ b/cw_bitcoin/test/locktime_test.dart
@@ -0,0 +1,32 @@
+import 'package:cw_bitcoin/locktime.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  group('antiFeeSnipingLocktime', () {
+    test('returns 0 when not synced', () {
+      expect(antiFeeSnipingLocktime(chainTip: 800000, synced: false), 0);
+    });
+
+    test('returns 0 when chainTip <= 0', () {
+      expect(antiFeeSnipingLocktime(chainTip: 0, synced: true), 0);
+    });
+
+    test('returns the current tip when synced', () {
+      expect(antiFeeSnipingLocktime(chainTip: 800000, synced: true), 800000);
+    });
+  });
+
+  group('locktimeToBytes', () {
+    test('encodes 0 as four zero bytes', () {
+      expect(locktimeToBytes(0), [0, 0, 0, 0]);
+    });
+
+    test('encodes a block height little-endian', () {
+      expect(locktimeToBytes(800000), [0x00, 0x35, 0x0C, 0x00]);
+    });
+
+    test('encodes max 32-bit value', () {
+      expect(locktimeToBytes(0xFFFFFFFF), [0xFF, 0xFF, 0xFF, 0xFF]);
+    });
+  });
+}