Fixes stale entity footnotes surviving Clear Cache. That action wipes self.characters/historical_figures/locations/terms - the entity lists our scan is built from - but our overlay had no way to know it happened, so the underline overlay and tap targets kept showing entity mentions from the book's now-discarded data until something unrelated (a settings toggle, opening the menu on the next book) happened to trigger a rescan.
Hooked Clear Cache the same way the existing init/getSubMenuItems hooks work: it now also clears our in-memory scan state (underlines, tap targets, matched-by-page buckets) and deletes the on-disk entity cache file, so a stale signature can't get reused later if a fresh fetch happens to reproduce the same entity names.
Scope: this only affected the explicit "Clear Cache" menu action. Normal book close/reopen and settings-toggle rescans were already correct.