Strip splice prototype bit when peering with ACINQ

LDK 0.2.2 renames the splice feature from prototype bit 155 to
production bit 63. ACINQ has already moved to bit 63 and their
eclair stack chokes when both bits are set on the same Init.

We need dual-advertise to keep talking to un-upgraded clients on
bit 155 once the LSP bumps, so hide bit 155 from ACINQ
specifically. ChannelManager's BaseMessageHandler impl of
provided_init_features already receives the counterparty pubkey
but was discarding it; start using it.

This is a no-op until the follow-up commit adds
set_splicing_production_optional(). Today only bit 155 is set,
and stripping it for ACINQ leaves nothing, which matches today's
state (no splice with ACINQ either way).

Refs MDK-799

Author: amackillop

lightning/src/ln/channelmanager.rs

@@ -13711,8 +13711,10 @@ where
 		provided_node_features(&self.config.read().unwrap())
 	}
 
-	fn provided_init_features(&self, _their_init_features: PublicKey) -> InitFeatures {
-		provided_init_features(&self.config.read().unwrap())
+	fn provided_init_features(&self, their_node_id: PublicKey) -> InitFeatures {
+		let mut features = provided_init_features(&self.config.read().unwrap());
+		strip_acinq_splice_prototype(&mut features, &their_node_id);
+		features
 	}
 
 	#[rustfmt::skip]
@@ -15701,6 +15703,25 @@ pub(crate) fn provided_channel_type_features(config: &UserConfig) -> ChannelType
 	ChannelTypeFeatures::from_init(&provided_init_features(config))
 }
 
+/// ACINQ mainnet node id. Eclair (ACINQ's stack) chokes on dual-advertise of
+/// the splice prototype (bit 155) and production (bit 63) feature bits, so we
+/// strip the prototype bit when peering with this specific node. ACINQ has
+/// already moved to bit 63 so they will accept splice via the production bit.
+/// See MDK-799.
+const ACINQ_MAINNET_NODE_ID: [u8; 33] = [
+	0x03, 0x86, 0x4e, 0xf0, 0x25, 0xfd, 0xe8, 0xfb, 0x58, 0x7d, 0x98, 0x91, 0x86, 0xce, 0x6a, 0x4a,
+	0x18, 0x68, 0x95, 0xee, 0x44, 0xa9, 0x26, 0xbf, 0xc3, 0x70, 0xe2, 0xc3, 0x66, 0x59, 0x7a, 0x3f,
+	0x8f,
+];
+
+/// If `their_node_id` is the ACINQ mainnet node, clear the splice prototype
+/// (bit 155) feature. See [`ACINQ_MAINNET_NODE_ID`] for context.
+fn strip_acinq_splice_prototype(features: &mut InitFeatures, their_node_id: &PublicKey) {
+	if their_node_id.serialize() == ACINQ_MAINNET_NODE_ID {
+		features.clear_splicing();
+	}
+}
+
 /// Fetches the set of [`InitFeatures`] flags that are provided by or required by
 /// [`ChannelManager`].
 pub fn provided_init_features(config: &UserConfig) -> InitFeatures {