Promotes the one unreleased changelog fragment into a 2.2.1 section and
bumps the project version. Mechanical release prep only; the tag and the
Hex publish stay with the operator.

- CHANGELOG.md: new `## [2.2.1] 2026-08-29` section carrying the
  fragment's Changed and Fixed entries verbatim, ordered per
  changelog.d/README.md.
- changelog.d/st-m2u8.md: deleted, promoted into the section above.
- mix.exs: @version 2.2.0 -> 2.2.1. The README install pin stays
  `~> 2.2`; a patch release does not move it.

Refs: st-i1bm