renepay: calculate fee and delay accumulation correctly.

1. When we add a shadow amount, we were using the wrong channel for
   the fee calculation.
2. Similarly, when calculating the delay amount.

The result is that we can get WIRE_INCORRECT_CLTV_EXPIRY repeatedly
from nodes.

Reported-by: https://github.com/Sjors
Fixes: #6620
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changlog-Experimental: Fixed: `renepay` handles ctlv correctly when it varies along a path.

plugins/renepay/pay_flow.c

@@ -172,8 +172,8 @@ static bool add_to_amounts(const struct gossmap *gossmap,
 	for (int i = num-2; i >= 0; i--) {
 		amounts[i] = amounts[i+1];
 		if (!amount_msat_add_fee(&amounts[i],
-					 flow_edge(f, i)->base_fee,
-					 flow_edge(f, i)->proportional_fee))
+					 flow_edge(f, i+1)->base_fee,
+					 flow_edge(f, i+1)->proportional_fee))
 			return false;
 	}
 
@@ -322,7 +322,7 @@ static void convert_and_attach_flows(struct payment *payment,
 		pf->cltv_delays[plen-1] = final_cltvs[i];
 		for (int j = (int)plen-2; j >= 0; j--) {
 			pf->cltv_delays[j] = pf->cltv_delays[j+1]
-				+ f->path[j]->half[f->dirs[j]].delay;
+				+ f->path[j+1]->half[f->dirs[j+1]].delay;
 		}
 		pf->amounts = tal_steal(pf, f->amounts);
 		pf->success_prob = f->success_prob;

tests/test_renepay.py

@@ -18,7 +18,6 @@ def test_simple(node_factory):
     assert details['destination'] == l2.info['id']
 
 
-@pytest.mark.xfail(strict=True)
 def test_direction_matters(node_factory):
     '''Make sure we use correct delay and fees for the direction we're going.'''
     l1, l2, l3 = node_factory.line_graph(3,