simln-lib: make the remaining selects biased

Two select! loops still polled their branches in tokio's default random order:
the payment-failure result send in produce_simulation_results, and the
interceptor-result drain in the SimNode HTLC forwarding path. Random branch
selection is a source of run-to-run non-determinism.

Make both biased so their poll order is fixed. Branch order is unchanged, so
behaviour is identical save for the now-deterministic ordering. The shutdown
listener already sits first in the failure-send loop, matching the
shutdown-first convention used elsewhere.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: carlaKC

simln-lib/src/lib.rs

@@ -1514,6 +1514,7 @@ async fn produce_simulation_results(
                     },
                     SimulationOutput::SendPaymentFailure(payment, result) => {
                         select! {
+                            biased;
                             _ = listener.clone() => {
                                 return Ok(());
                             },

simln-lib/src/sim_node.rs

@@ -964,6 +964,7 @@ async fn handle_intercepted_htlc(
     let mut interceptor_failure = None;
     'get_resp: loop {
         tokio::select! {
+            biased;
             res = intercepts.join_next() => {
                 let res = match res {
                     Some(res) => res,