Bumps the version carriers to 0.15.0: mix.exs's @version, the
compiler's @compiler_version, and the README install pin.

Promotes the ten changelog.d fragments into a 0.15.0 section
dated 2026-09-02, grouped Added and Fixed, and deletes them.
The ADR, docs 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-ya0y