Release prep only: the tag, the Hex publish and the GitHub release stay
the operator's. A patch, because the one entry is a fix with no public
API change.

- mix.exs: @version 0.6.0 -> 0.6.1
- CHANGELOG.md: new [0.6.1] 2026-09-03 section, the sob-axb fragment's
  Fixed bullet carried over byte for byte
- changelog.d/sob-axb.md: deleted, promoted into the section above
- README.md: install pin untouched, `~> 0.6` already admits 0.6.1

Refs: sob-axb