Fix CSV export corruption; add score/grade columns, filtered downloads, cross-sell

CSV export was silently corrupted: export() assumed
\core\dataformat::download_data() terminates execution, but it doesn't in
this Moodle version - control returned to mod/quiz/report.php's dispatcher,
which called $OUTPUT->footer() without a matching header(), throwing a
coding_exception whose error page landed appended to the downloaded file.
Fixed with an explicit exit after the download.

Also: split the combined "obtained / max" score into separate raw-numeric
CSV columns (Puntaje obtenido, Calificación obtenida - the latter rescaled
onto the quiz's own configured grade via sumgrades, not just raw marks);
the CSV download now respects whatever search text and risk/strong filter
chip the teacher has active on screen instead of always exporting everyone;
added a brand mark + discreet cross-sell footer linking to Asistente IA and
WhatsApp Notify while this report is still free/trial-only.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
