chore: prepare 0.6.1rc2

[utensil]

Summary

• Bump galgebra/_version.py from 0.6.1rc1 to 0.6.1rc2.
• Add the missing 0.6.1 changelog entry for docs: fix DOI badge and citation metadata #599; fix: avoid slow simplification during multivector display #598 is already documented.
• Preserve the final-only 0.6.1 release marker, per the release runbook.

Verification

• uv run --python 3.12 python -c "from galgebra._version import __version__; assert __version__ == '0.6.1rc2'"
• git diff --check
• Confirmed the nbval sanitizer covers rcN version suffixes.

Release plan

After this PR is squash-merged, create and validate the v0.6.1rc2 prerelease before preparing the final 0.6.1 bump.

Part of #588.

[utensil]

0.6.1rc2 will include the post-rc1 changes #598 and #599. #598 adds a display-scoped mitigation for the simplification regression introduced upstream in SymPy 1.13; the curvilinear-coordinate example also retains the earlier #590 workaround.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.90%. Comparing base (70efd80) to head (277ffb1).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #600   +/-   ##
=======================================
  Coverage   73.90%   73.90%           
=======================================
  Files          23       23           
  Lines        4518     4518           
=======================================
  Hits         3339     3339           
  Misses       1179     1179           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.