Add reason and splice parameters to splice failure events

An application handling SpliceNegotiationFailed had nothing to act on:
the event did not say why the splice failed, nor what the failed call
had attempted. Both matter for deciding what to do next — a fee bump
lost to a disconnect can simply be re-issued, while the splice it meant
to bump may still confirm at the prior feerate.

Attach a reason, mapped from LDK's NegotiationFailureReason onto an
ldk-node-owned enum so the event's serialization and bindings do not
change with LDK's, and the parameters of the originating API call,
taken from the persisted splice intent when the failure identifies it.
Both fields are optional and serialized as odd TLVs: events written by
LDK Node v0.7 read back as None, and v0.7 readers ignore the new
fields.

Developed with assistance from Claude Code.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: jkczyz

src/channel/mod.rs

@@ -504,6 +504,11 @@ pub(crate) struct FailureSettlement<'a> {
 }
 
 impl FailureSettlement<'_> {
+	/// The parameters of the API call behind the splice the failure identifies, if any.
+	pub(crate) fn originating_kind(&self) -> Option<&SpliceKind> {
+		self.matched.as_ref().map(|(_, intent)| &intent.kind)
+	}
+
 	/// Settles the snapshotted intent, if any. Call only once the user-facing failure event is
 	/// durably queued.
 	pub(crate) async fn settle(self) {