fix(lnd): SendPaymentV2 needs an explicit fee budget — absent means ZERO

v1.8.9's move to Router.SendPaymentV2 shipped without fee_limit_sat,
and the v2 route treats an ABSENT fee limit as zero allowed fees.
Every real route carries a routing fee (the 2-hop route here: 1.5
sats), so the pathfinder rejected them all and the wallet answered
"No route to the recipient" on EVERY send — all day, on healthy
channels with plenty of liquidity both ways.

The router debug log makes it unambiguous:
  wallet payment (v1.8.9 backend): fee_limit=0 mSAT     -> no route
  same payment by hand (lncli --fee_limit=100): fee_limit=100000 mSAT -> settles in 0.65s

My earlier "pipeline verified" claim was wrong — the manual lncli
verification set a fee limit by hand and masked this exact bug. The
400k that succeeded this morning went through the pre-update backend
on the pre-update LND.

Payments now carry lncli's own default budget — the payment amount
(100%), preferring the payer-supplied amount for zero-value invoices
and the invoice's own amount otherwise, with a nominal floor so the
limit can never be zero. Unit-pinned so it cannot regress.

core/archipelago/src/api/rpc/lnd/payments.rs

@@ -42,6 +42,21 @@ fn json_i64(value: &serde_json::Value, key: &str) -> Option<i64> {
     })
 }
 
+/// Fee budget for a send, matching lncli's own default: the payment amount
+/// (100%). Zero-amount invoices take the payer-supplied amount; fixed invoices
+/// take the invoice's own amount. Falls back to a nominal 1,000 sats only when
+/// both are somehow absent — the limit must never be left at LND's zero
+/// default, which rejects every fee-carrying route as "no route".
+fn fee_limit_sats(amount_sats: Option<u64>, decoded_amt: i64) -> i64 {
+    if let Some(amt) = amount_sats {
+        return amt as i64;
+    }
+    if decoded_amt > 0 {
+        return decoded_amt;
+    }
+    1_000
+}
+
 impl RpcHandler {
     /// Pay a Lightning invoice.
     pub(in crate::api::rpc) async fn handle_lnd_payinvoice(
@@ -107,6 +122,14 @@ impl RpcHandler {
             // enough, and it makes grpc-gateway's response a single JSON value.
             "no_inflight_updates": true,
             "timeout_seconds": 120,
+            // Router.SendPaymentV2 treats an ABSENT fee limit as ZERO — every
+            // real route carries a routing fee, so the pathfinder rejects
+            // them all and the wallet gets "No route to the recipient" on
+            // every send (fleet-wide, 2026-09-01: the v1.8.9 switch to the v2
+            // route shipped without this, and a manual lncli test that set
+            // --fee_limit masked it). lncli's own default is the payment
+            // amount (100%), which is what we send here.
+            "fee_limit_sat": fee_limit_sats(amount_sats, decoded_amt),
         });
         if let Some(amt) = amount_sats {
             pay_body["amt"] = serde_json::json!(amt.to_string());
@@ -550,4 +573,15 @@ mod tests {
             "Insufficient channel balance"
         );
     }
+
+    #[test]
+    fn fee_limit_never_falls_back_to_zero() {
+        // SendPaymentV2 defaults an ABSENT fee limit to zero — which rejects
+        // every fee-carrying route as "no route". The budget must always be
+        // positive: the payer-supplied amount for zero-amount invoices, the
+        // invoice's own amount otherwise.
+        assert_eq!(fee_limit_sats(Some(20_000), 0), 20_000);
+        assert_eq!(fee_limit_sats(None, 20_000), 20_000);
+        assert_eq!(fee_limit_sats(None, 0), 1_000);
+    }
 }