Pre-fix to_dict omitted the required field content, so _load's
CacheEntry(**entry_data) raised TypeError on every reload and the
blanket except silently discarded the whole cache on every restart.
Frozen regression contract tests/test_token_optimizer_cache.py
(3 tests: save_persists_content, roundtrip_restores_content,
corrupt_storage_degrades_to_empty_cache). Full suite 2250/0.
