Update changelog and citation for v26.3.1

[rtimms]

Summary

• Moves the Fix load_custom_model when base class package is unavailable #5441 and Fix serialization in 2D FVM #5434 bug fix entries from Unreleased into a new v26.3.1 section in CHANGELOG.md
• Updates CITATION.cff version to 26.3.1

This is the main changelog update for the v26.3.1 patch release, as per the release workflow (step 7). The release tag will be created separately from the release/v26.3.1 branch.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.31%. Comparing base (d46beb3) to head (67906dd).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5443   +/-   ##
=======================================
  Coverage   98.31%   98.31%           
=======================================
  Files         336      336           
  Lines       30700    30699    -1     
=======================================
  Hits        30183    30183           
+ Misses        517      516    -1     

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