Add local-day activity summaries derived from UTC hour observations using
the system timezone, with durable storage in a new local_days table.
The collector derives local-day read/write totals after UTC aggregation;
the TUI displays them with timezone, completeness state, and coverage.
Schema: bump SCHEMA_VERSION to 3, add local_days table (migration 2→3
is pure addition, idempotent, preserves newer-schema refusal).
- 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