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

src/fenris/derive.py

@@ -175,7 +175,8 @@ def _upsert_hour_observation(
     """Insert or update an hour observation."""
     # Check if hour exists
     existing = conn.execute(
-        "SELECT id, bytes_written_delta, sample_count FROM hour_observations WHERE hour = ?",
+        "SELECT id, bytes_written_delta, bytes_read_delta, sample_count "
+        "FROM hour_observations WHERE hour = ?",
         (hour_key,),
     ).fetchone()
 
@@ -198,10 +199,13 @@ def _upsert_hour_observation(
     else:
         # Merge: accumulate bytes and sample count
         new_bw = existing[1] + bw_delta
-        new_samples = existing[2] + sample_count
+        new_br = existing[2] + br_delta
+        new_samples = existing[3] + sample_count
         conn.execute(
-            "UPDATE hour_observations SET bytes_written_delta = ?, sample_count = ? WHERE id = ?",
-            (new_bw, new_samples, existing[0]),
+            "UPDATE hour_observations "
+            "SET bytes_written_delta = ?, bytes_read_delta = ?, sample_count = ? "
+            "WHERE id = ?",
+            (new_bw, new_br, new_samples, existing[0]),
         )
     conn.commit()