perf: preallocate omitted marker buffer

Reserve the final omitted-marker capacity before extending from the iterator to avoid incremental growth in selective disclosure construction.

Co-Authored-By: OpenAI Codex <noreply@openai.com>

Author: vincenzopalazzo

lightning/src/offers/merkle.rs

@@ -389,7 +389,10 @@ pub(super) fn compute_selective_disclosure<'a>(
 	if tlv_data.is_empty() {
 		return Err(SelectiveDisclosureError::EmptyTlvStream);
 	}
-	let omitted_markers: Vec<_> = compute_omitted_markers(tlv_data.iter()).collect();
+	let num_omitted_markers =
+		tlv_data.iter().filter(|data| !data.is_included && data.tlv_type != 0).count();
+	let mut omitted_markers = Vec::with_capacity(num_omitted_markers);
+	omitted_markers.extend(compute_omitted_markers(tlv_data.iter()));
 	let (merkle_root, missing_hashes) = build_tree_with_disclosure(&tlv_data, &branch_tag);
 
 	Ok(SelectiveDisclosure { leaf_hashes, omitted_markers, missing_hashes, merkle_root })