Validation bypass via basename-keyed schema cache

Cache confusion in check-jsonschema before 0.30.0 allows attackers to bypass validation by priming the cache with a malicious remote schema. BoundCacheDownloader derives each cache filename only from the URL basename, so distinct origins ending in the same name share one cache entry. Exploitation requires a user or automation to run the tool against the attacker's schema URL before a trusted same-basename URL is checked, causing data that should fail validation to pass.