Ignore *.rois.json sidecars and fix stale test_data/ paths in .gitignore

GUI writes .rois.json sidecars next to scan files when a file is opened
interactively; these are ephemeral user state that should never be committed.
Also fixes dead data/ references (folder was renamed to test_data/).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: jacobson30-bot

.gitignore

@@ -9,8 +9,10 @@ __pycache__/
 build/
 dist/
 
-data/output_png/*
-!data/output_png/.gitkeep
+test_data/output_png/*
+!test_data/output_png/.gitkeep
 
-data/output_sxm/*
-!data/output_sxm/.gitkeep
+test_data/output_sxm/*
+!test_data/output_sxm/.gitkeep
+
+*.rois.json