docs(CHANGES): refresh release notes for May 2026

[tony]

Summary

• Rewrite the full changelog into the newer release-note style used across the gp-libs docs stack
• Expand release entries into user-facing deliverables with lead paragraphs and prose sections
• Add current MyST links for live docs/API targets while leaving obsolete early helper APIs unlinked

Validation

• uv run sphinx-build -b dirhtml -n -W --keep-going docs docs/_build/html
• uv run ruff check .
• uv run ruff format --check .
• uv run mypy .
• uv run py.test --basetemp=/home/d/.cache/g-pytest-basetemp

Note: plain uv run py.test fails in this container because /tmp/.git exists, so pytest tmp dirs under /tmp are detected as inside a git repo. The suite passes when pytest's base temp is outside /tmp.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.89%. Comparing base (17a9096) to head (b470da9).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #56   +/-   ##
=======================================
  Coverage   96.89%   96.89%           
=======================================
  Files           5        5           
  Lines         129      129           
=======================================
  Hits          125      125           
  Misses          4        4           

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