Cache keys now include a schema version prefix so that world-schema
changes automatically invalidate stale cached extraction results.
Previously two incompatible schema versions would share the same
cache key if URL/selectors/env matched, silently returning stale data.
WORLD_CACHE_SCHEMA_VERSION exported publicly so consumers can
increment it when making schema-incompatible changes to extraction.
658 tests pass.
John Dvorak
a49d6008a4