fuzz: add single-channel MPP test to chanmon_consistency

Add fuzz command 0x74 to test MPP payments where the recipient receives
multiple parts over a single channel. This complements the existing MPP
tests which use different channels for each part.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: joostjager

fuzz/src/chanmon_consistency.rs

@@ -2083,6 +2083,8 @@ pub fn do_test<Out: Output>(data: &[u8], underlying_out: Out, anchors: bool) {
 			0x72 => send_mpp_hop(0, 1, &chan_ab_scids, 2, &chan_bc_scids, 1_000_000, &mut p_ctr),
 			// 0x73: MPP 0->1->2, multi channels on second hop (B-C)
 			0x73 => send_mpp_hop(0, 1, &[chan_a], 2, &chan_bc_scids, 1_000_000, &mut p_ctr),
+			// 0x74: direct MPP from 0 to 1, multi parts over single channel
+			0x74 => send_mpp_direct(0, 1, &[chan_a, chan_a, chan_a], 1_000_000, &mut p_ctr),
 
 			0x80 => {
 				let mut max_feerate = last_htlc_clear_fee_a;