Dual-advertise splice prototype and production feature bits

LDK 0.2.2 will rename the splice feature from prototype bit 155
to production bit 63. Setting both bits during the migration
window keeps splice working with three peer cohorts on the same
LSP binary:

  - un-upgraded clients on bit 155 only,
  - upgraded clients and ACINQ on bit 63,
  - upgraded clients that still see bit 155 (no harm).

ACINQ is the exception: their eclair stack chokes when both bits
are set, so the previous commit strips bit 155 from outbound
Inits to ACINQ specifically.

Sunset path: when all clients have upgraded, drop the
set_splicing_optional() call and re-bump.

Refs MDK-799

Author: amackillop

lightning/src/ln/channelmanager.rs

@@ -15669,6 +15669,11 @@ where
 
 /// Fetches the set of [`NodeFeatures`] flags that are provided by or required by
 /// [`ChannelManager`].
+///
+/// Note: the ACINQ splice-prototype carve-out applied in
+/// [`BaseMessageHandler::provided_init_features`] is intentionally *not*
+/// applied here. The issue with dual-advertise is in `Init`, not in
+/// gossip `node_announcement`s, and gossip is broadcast not per-peer.
 pub(crate) fn provided_node_features(config: &UserConfig) -> NodeFeatures {
 	let mut node_features = provided_init_features(config).to_context();
 	node_features.set_keysend_optional();
@@ -15746,6 +15751,9 @@ pub fn provided_init_features(config: &UserConfig) -> InitFeatures {
 	features.set_simple_close_optional();
 	features.set_quiescence_optional();
 	features.set_splicing_optional();
+	// Dual-advertise alongside the prototype bit during the LDK 0.2.2 migration.
+	// Stripped per-peer for ACINQ in the BaseMessageHandler impl above. See MDK-799.
+	features.set_splicing_production_optional();
 
 	if config.channel_handshake_config.negotiate_anchors_zero_fee_htlc_tx {
 		features.set_anchors_zero_fee_htlc_tx_optional();