Release 2.3.1 takes main by a merge before the release PR #256

release/2.3.1 was cut from v2.3.0, and main has since opened 2.4.0 (#179: Version 2.4.0, a ## 2.4.0 — Unreleased section). The release PR into main conflicts on both, and the release workflow checks the tag against Version, so the merge commit that takes v2.3.1 must say 2.3.1.

Per decision 94, a release branch that falls behind main takes main by a merge. The merge keeps the release side of both files, so its tree is release/2.3.1's exactly. After the tag, CONTRIBUTING's step 5 opens 2.4.0 on main again, with the package-validation baseline at 2.3.1, and main is merged into release/2.4.0.