Honor the per-peer fee policy at the skim site

The forwarding skim now reads the peer's granted policy from the SCID store
instead of hard-coding Flat(Standard). A peer with no grant resolves to
Standard, so the skim is byte-for-byte the historical 2% for everyone the
issuer set has not waived. This is the read side that makes the persisted
grant actually take effect; before it, a verified ZeroFee was stored and
then ignored.

The lookup is hoisted out of the per-HTLC loop because the policy is keyed
by peer, not by HTLC, so it is one store read per batch rather than one per
forward.

A zero skim is now two distinct cases, so the log is split. ZeroFee is the
expected path and logs at info. Any other tier skimming nothing means the
fee would have consumed the whole HTLC, which keeps the error-level log:
a forward of the full amount that we expected to take a cut on. Previously
ZeroFee was unreachable, so the single error log was correct; now that a
grant can legitimately waive the fee, the error would be noise.

Author: amackillop

lightning-liquidity/src/lsps4/service.rs

@@ -466,6 +466,14 @@ where
 			skimmed_fee_msat: u64,
 		}
 
+		// The peer's granted policy, looked up once. Peers with no grant resolve to Standard, so
+		// the skim matches the historical 2% for everyone the issuer set hasn't waived.
+		let policy = self
+			.scid_store
+			.get_policy(&their_node_id)
+			.unwrap_or(FeePolicy::Flat(FeeTier::Standard));
+		let is_zero_fee = matches!(policy, FeePolicy::Flat(FeeTier::ZeroFee));
+
 		let mut computed_htlcs: Vec<ComputedHtlc> = htlcs
 			.drain(..)
 			.map(|htlc| {
@@ -476,18 +484,26 @@ where
 
 				let htlc_id = htlc.id();
 				let skimmed_fee_msat = resolve_skim(
-					&FeePolicy::Flat(FeeTier::Standard),
+					&policy,
 					expected_outbound_msat,
 					self.config.forwarding_fee_proportional_millionths,
 				);
 				if skimmed_fee_msat == 0 {
-					// The policy here is always Flat(Standard), so a zero skim can only mean the
-					// fee would have consumed the entire HTLC; it is never a ZeroFee waiver yet.
-					log_error!(
-						self.logger,
-						"Standard skim would have consumed the entire HTLC {:?}; forwarding the full amount.",
-						htlc_id
-					);
+					if is_zero_fee {
+						log_info!(
+							self.logger,
+							"Zero-fee policy for HTLC {:?}; forwarding the full amount.",
+							htlc_id
+						);
+					} else {
+						// A non-zero-fee tier skimmed nothing only because the fee would have
+						// consumed the entire HTLC; forward it intact rather than break it.
+						log_error!(
+							self.logger,
+							"Skim would have consumed the entire HTLC {:?}; forwarding the full amount.",
+							htlc_id
+						);
+					}
 				}
 
 				let amount_to_forward_msat = expected_outbound_msat.saturating_sub(skimmed_fee_msat);