Publish consistent measured drive activity (#89)

Repair the collection-to-display path so each successful acquisition
publishes correct read/write deltas through the observation store and
visible dashboard.

Fixes:
- derive.py: accumulate bytes_read_delta on same-hour hour_observation
  merge (was silently dropped)
- collector.py: rebuild day_aggregates from hour observations after
  each collection run (previously only populated for cross-hour intervals)
- day_aggregate.py: add persist_day_aggregate upsert helper
- tui.py: query and display both read and write deltas in daily and
  hourly readouts, constrained summaries, and graph data queries

Tests:
- Add 14 integration tests (test_measured_activity.py) exercising the
  full collector→store→reader→display path with real fixtures and
  injected time
- Update constrained-layout assertion to match new W/R format

Closes #89

tests/test_tui.py

@@ -1179,7 +1179,7 @@ class TestConstrainedLayout:
             summary_text = str(summary.render())
             assert "Graph needs ≥80×24" in summary_text
             assert "days" in summary_text
-            assert "GB total" in summary_text
+            assert "GB" in summary_text
 
     @pytest.mark.asyncio
     async def test_constrained_below_24_height(self, tmp_path):