Usage and time observation handoff (WS-041 → WS-043)

WS-041 records historical usage and wall-clock observations. WS-043 may consume them as covered history for forecasting, but must not treat any cumulative-duration field as estimated remaining time.

Surfaces

• Core: awr_core::workstream_usage — dedup, cost totals, counter deltas, allocations, coverage reports, corrections, usage_observation_handoff, refuse_eta_from_cumulative_duration.
• Store (schema 13 / usage_time): persisted receipts, corrections, allocation records, counter snapshots, execution intervals, idempotent ingest receipts.
• Runtime: attested ingest/query APIs and usage_observation_for_ws043.

Invariants for WS-043

1. UsageTimeObservationHandoff.is_historical_observation is always true.
2. is_not_estimated_remaining_time is always true; refuse ETA labels on cumulative duration via refuse_eta_from_cumulative_duration.
3. Coverage (call_coverage / time_coverage) is optional only when the expected denominator is known; unknown expected never invents 100% coverage.
4. Actual, API-equivalent, and unknown costs remain separate from coverage.
5. Parallel observed_wall_clock_ms is not equal to observed_execution_ms when intervals overlap.

This note is a handoff contract, not a forecasting implementation.

See also eta-checkpoints.md for the WS-043 forecasting surface that consumes this handoff.