Bumps the version carriers to 0.14.0: mix.exs's @version, the
compiler's @compiler_version, and the README install pin.

Promotes the four changelog.d fragments into a 0.14.0 section
dated 2026-09-01, grouped Added, Changed and Fixed, and deletes
them. The ADR and record-only pull requests in the release carry
no fragment by design.

No tag and no publish - those stay the maintainer's.

Refs: sb-qx6k