docs: add LP write speed improvement to release notes

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: FBumann

doc/release_notes.rst

@@ -6,6 +6,7 @@ Upcoming Version
 
 * Fix docs (pick highs solver)
 * Add the `sphinx-copybutton` to the documentation
+* Speed up LP file writing by 2-2.7x on large models through Polars streaming engine, join-based constraint assembly, and reduced per-constraint overhead
 
 Version 0.6.1
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