- Schema migration 1-2: add segment_id to samples, unattributed bytes to day_aggregates
- Collector now derives hour observations and day aggregates from sample pairs
- Cross-hour deltas tracked as unattributed (no proportional allocation)
- Display states: 0 samples -> awaiting first, 1 sample -> awaiting another
- Monitoring period ensured open on each collection run
- Derivation failures preserve prior history
Closes#73