dfa: make slow-bail progress tracking direction-aware in reverse scans

[jmestwa-coder]

This patch fixes progress tracking in the DFA slow-bail path for reverse scans.

The existing logic measures progress as p - resetp, which is correct for forward traversal but not for reverse traversal, where p moves backward. In that case, the difference becomes negative after progress and can turn into a large unsigned value when cast to size_t, preventing the intended bailout check from triggering.