Reorganize 1.3.0 release log by importance with headings

[derek73]

Summary

• Group the unreleased 1.3.0 release log entries under four headings: Breaking Changes & Deprecations, Behavior Changes, New Features, and Bug Fixes
• Within each group, order items with the most impactful/relevant first (e.g. deprecations and default-on behavior changes before cosmetic fixes)
• All entries are moved verbatim, nothing was reworded or dropped

Test plan

• Verified the file still parses as valid RST via docutils
• Confirmed in rendered HTML that the bold headings correctly group the nested bullet list without breaking list structure

🤖 Generated with Claude Code

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.52%. Comparing base (9364241) to head (26be15b).
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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #248   +/-   ##
=======================================
  Coverage   97.52%   97.52%           
=======================================
  Files          13       13           
  Lines         888      888           
=======================================
  Hits          866      866           
  Misses         22       22           

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