Problem: after an install or update, the on-disk cache may contain data
written by an older version of the plugin whose format no longer matches
what the current code expects.
Solution: embed a CACHE_VERSION in every saved cache file. On load, if
the stored version is missing or differs from the current one, wipe the
cache and rewrite it. Corrupt (non-decodable) cache files are handled
the same way instead of only logging an error.