Debug image output

Every call to extractAnswers attempts to create a JPEG overlay showing contours that were accepted as filled bubbles. The current output path is:

./data/debug/debug_grid_<original-file-name>_<current-time-millis>.jpeg

The path is relative to the running process's working directory. The implementation does not create the directory, sanitize the original file name, limit output size, rotate files, or delete old images. Write failures are caught and ignored.

This is a side effect of both API processing and OMR tests, not an opt-in diagnostic feature. Treat the directory as potentially containing submitted exam imagery and do not assume a retention policy or access controls exist. Controlled debug output and retention are not currently implemented.