fuzz: gate splice opcodes on cfg(splicing)

Keep the splice opcodes in chanmon_consistency available
only when the crate is built with cfg(splicing). When
splicing is disabled, return early from those opcode
handlers instead of calling splice helpers that are not
compiled in.

Add cfg(splicing) to fuzz/Cargo.toml check-cfg so the
guarded code builds cleanly in the fuzz crate.

AI tools were used in preparing this commit.

Author: joostjager

fuzz/Cargo.toml

@@ -62,4 +62,5 @@ check-cfg = [
     "cfg(fuzzing)",
     "cfg(secp256k1_fuzz)",
     "cfg(hashes_fuzz)",
+    "cfg(splicing)",
 ]

fuzz/src/chanmon_consistency.rs

@@ -1333,7 +1333,9 @@ pub fn do_test<Out: Output + MaybeSend + MaybeSync>(data: &[u8], out: Out) {
 	let (node_c, mut monitor_c, keys_manager_c, logger_c) = make_node!(2, fee_est_c, broadcast_c);
 
 	let mut nodes = [node_a, node_b, node_c];
+	#[allow(unused_variables)]
 	let loggers = [logger_a, logger_b, logger_c];
+	#[allow(unused_variables)]
 	let fee_estimators = [Arc::clone(&fee_est_a), Arc::clone(&fee_est_b), Arc::clone(&fee_est_c)];
 
 	// Connect peers first, then create channels
@@ -2426,52 +2428,76 @@ pub fn do_test<Out: Output + MaybeSend + MaybeSync>(data: &[u8], out: Out) {
 			},
 
 			0xa0 => {
+				if !cfg!(splicing) {
+					test_return!();
+				}
 				let cp_node_id = nodes[1].get_our_node_id();
 				let wallet = WalletSync::new(&wallets[0], Arc::clone(&loggers[0]));
 				let feerate_sat_per_kw = fee_estimators[0].feerate_sat_per_kw();
 				splice_in(&nodes[0], &cp_node_id, &chan_a_id, &wallet, feerate_sat_per_kw);
 			},
 			0xa1 => {
+				if !cfg!(splicing) {
+					test_return!();
+				}
 				let cp_node_id = nodes[0].get_our_node_id();
 				let wallet = WalletSync::new(&wallets[1], Arc::clone(&loggers[1]));
 				let feerate_sat_per_kw = fee_estimators[1].feerate_sat_per_kw();
 				splice_in(&nodes[1], &cp_node_id, &chan_a_id, &wallet, feerate_sat_per_kw);
 			},
 			0xa2 => {
+				if !cfg!(splicing) {
+					test_return!();
+				}
 				let cp_node_id = nodes[2].get_our_node_id();
 				let wallet = WalletSync::new(&wallets[1], Arc::clone(&loggers[1]));
 				let feerate_sat_per_kw = fee_estimators[1].feerate_sat_per_kw();
 				splice_in(&nodes[1], &cp_node_id, &chan_b_id, &wallet, feerate_sat_per_kw);
 			},
 			0xa3 => {
+				if !cfg!(splicing) {
+					test_return!();
+				}
 				let cp_node_id = nodes[1].get_our_node_id();
 				let wallet = WalletSync::new(&wallets[2], Arc::clone(&loggers[2]));
 				let feerate_sat_per_kw = fee_estimators[2].feerate_sat_per_kw();
 				splice_in(&nodes[2], &cp_node_id, &chan_b_id, &wallet, feerate_sat_per_kw);
 			},
 
 			0xa4 => {
+				if !cfg!(splicing) {
+					test_return!();
+				}
 				let cp_node_id = nodes[1].get_our_node_id();
 				let wallet = &wallets[0];
 				let logger = Arc::clone(&loggers[0]);
 				let feerate_sat_per_kw = fee_estimators[0].feerate_sat_per_kw();
 				splice_out(&nodes[0], &cp_node_id, &chan_a_id, wallet, logger, feerate_sat_per_kw);
 			},
 			0xa5 => {
+				if !cfg!(splicing) {
+					test_return!();
+				}
 				let cp_node_id = nodes[0].get_our_node_id();
 				let wallet = &wallets[1];
 				let logger = Arc::clone(&loggers[1]);
 				let feerate_sat_per_kw = fee_estimators[1].feerate_sat_per_kw();
 				splice_out(&nodes[1], &cp_node_id, &chan_a_id, wallet, logger, feerate_sat_per_kw);
 			},
 			0xa6 => {
+				if !cfg!(splicing) {
+					test_return!();
+				}
 				let cp_node_id = nodes[2].get_our_node_id();
 				let wallet = &wallets[1];
 				let logger = Arc::clone(&loggers[1]);
 				let feerate_sat_per_kw = fee_estimators[1].feerate_sat_per_kw();
 				splice_out(&nodes[1], &cp_node_id, &chan_b_id, wallet, logger, feerate_sat_per_kw);
 			},
 			0xa7 => {
+				if !cfg!(splicing) {
+					test_return!();
+				}
 				let cp_node_id = nodes[1].get_our_node_id();
 				let wallet = &wallets[2];
 				let logger = Arc::clone(&loggers[2]);