docs(changelog): consolidate duplicate Fixed/Added sections in 0.2.4b1

  The unreleased section accumulated two ### Fixed and two ### Added
  headers from successive PRs each adding their own subsection instead
  of appending under the existing one. Merge into one section per type,
  in canonical Keep-a-Changelog order (Added / Changed / Fixed /
  Security). All 68 entries preserved verbatim — no content change.