Keep the MixMHCpred3 comparison material off the Hub

benchmark/ is two publisher supplementary .xlsx files -- copyrighted, not ours to
redistribute -- plus a training dump, and its only provenance is three bare URLs in
mixmhcpred3_ref.md. That does not meet the SOURCES.md standard the rest of this repo now
holds. Kept on disk, gitignored, alongside the existing raw/ rule.

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

.gitignore

@@ -7,6 +7,12 @@ src/
 # raw downloads (never pushed)
 raw/
 
+# MixMHCpred3 comparison material (never pushed). Two of the four files are publisher
+# supplementary .xlsx — copyrighted, not ours to redistribute — and the directory's only
+# provenance is three bare URLs in benchmark/mixmhcpred3_ref.md, which does not meet the
+# SOURCES.md standard the rest of this repo now holds. Kept on disk, off the Hub.
+benchmark/
+
 # private data (never pushed) — real patient cohort
 gamaleya/
 # NB: tcga/ is PUBLIC (neoantigen predictions derived from open-access TCGA somatic +