fuzz: assert no stuck payments in chanmon_consistency

After settling all state at 0xff, verify that pending_payments is empty
for all nodes. If payments remain stuck indefinitely, that indicates a
bug in payment resolution.

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

Author: joostjager

fuzz/src/chanmon_consistency.rs

@@ -2548,6 +2548,16 @@ pub fn do_test<Out: Output>(data: &[u8], underlying_out: Out, anchors: bool) {
 
 				process_all_events!();
 
+				// Verify no payments are stuck - all should have resolved
+				for (idx, pending) in pending_payments.borrow().iter().enumerate() {
+					assert!(
+						pending.is_empty(),
+						"Node {} has {} stuck pending payments after settling all state",
+						idx,
+						pending.len()
+					);
+				}
+
 				// Finally, make sure that at least one end of each channel can make a substantial payment
 				for &scid in &chan_ab_scids {
 					assert!(