Rewrite changes.txt as CHANGES.md

[fingolfin]

Restructure the change log in the style used by other packages: newest version first, one section per release with its release date from the git tags, terse bullets. Correct function and file names that did not match the code, and drop the duplicated per-file recap of the 3.0.0 changes. Add bare dated headings for 2.4.0 to 2.5.4, for which no notes exist.

Assisted-by: Claude Code (Opus 5)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.05%. Comparing base (ab45a4c) to head (2019418).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #35   +/-   ##
=======================================
  Coverage   94.05%   94.05%           
=======================================
  Files          62       62           
  Lines       26624    26624           
=======================================
  Hits        25040    25040           
  Misses       1584     1584           

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[fingolfin]

@margollo this format is a bit closer to what other packages use and is perhaps a bit easier to read, esp. when the Markdown is rendered to HTML

[margollo]

Very nice, thanks!