Use inbound SCID alias for blinded path creation

When we generate a compact blinded path to ourselves, we currently
use the real SCID of our channels to tell our peers where to
forward to. This is fine for short-lived blinded paths where we
expect the channel to still be around, but post-splicing this may
spuriously invalidate blinded paths just because we spliced.

Instead, here, we default to using inbound SCID aliases where
possible. We also avoid one more reference to the channel's
internal `FundingContext` in `channelmanager.rs`.

Author: TheBlueMatt

lightning/src/ln/channel.rs

@@ -9403,6 +9403,12 @@ where
 		false
 	}
 
+	/// Gets the latest inbound SCID alias from our peer, or if none exists, the channel's real
+	/// SCID.
+	pub fn get_inbound_scid(&self) -> Option<u64> {
+		self.context.latest_inbound_scid_alias.or(self.funding.get_short_channel_id())
+	}
+
 	/// Returns true if our channel_ready has been sent
 	pub fn is_our_channel_ready(&self) -> bool {
 		matches!(self.context.channel_state, ChannelState::AwaitingChannelReady(flags) if flags.is_set(AwaitingChannelReadyFlags::OUR_CHANNEL_READY))

lightning/src/ln/channelmanager.rs

@@ -11524,8 +11524,9 @@ where
 					.channel_by_id
 					.iter()
 					.filter(|(_, channel)| channel.context().is_usable())
-					.min_by_key(|(_, channel)| channel.context().channel_creation_height)
-					.and_then(|(_, channel)| channel.funding().get_short_channel_id()),
+					.filter_map(|(_, channel)| channel.as_funded())
+					.min_by_key(|funded_channel| funded_channel.context.channel_creation_height)
+					.and_then(|funded_channel| funded_channel.get_inbound_scid()),
 			})
 			.collect::<Vec<_>>()
 	}