fix: report incomplete export writes

[sit-d]

Summary

• Track write failures while exporting stable logistic R tables.
• Check both CSV/INC formatted writes and close results before showing the success dialog.
• Report an export error if the files cannot be fully written.

Verification

• git diff --check HEAD~1..HEAD -- iterator_gdi.c
• Static export-path check confirmed formatted writes and file closes are checked before success is reported.

Notes

• Related: fix: restore only visible iterator windows #2 and fix: clean up DIB allocation failures #4 also touch iterator_gdi.c; this change is limited to the stable logistic R export path.
• Windows build validation was not run.