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/day_aggregate.py

@@ -163,3 +163,41 @@ def derive_all_days(conn: sqlite3.Connection) -> list[DayAggregate]:
         if agg is not None:
             results.append(agg)
     return results
+
+
+def persist_day_aggregate(conn: sqlite3.Connection, agg: DayAggregate) -> None:
+    """Upsert a derived day aggregate into the day_aggregates table.
+
+    Merges attributed bytes from hour observations with any existing
+    unattributed cross-hour evidence already stored for this day.
+    Caller must manage transactions and commits.
+    """
+    existing = conn.execute(
+        "SELECT id FROM day_aggregates WHERE day = ?",
+        (agg.day,),
+    ).fetchone()
+
+    if existing is None:
+        conn.execute(
+            "INSERT INTO day_aggregates "
+            "(day, active_seconds, idle_seconds, powered_off_seconds, unknown_seconds, "
+            " bytes_written_delta, bytes_read_delta, sample_count, coverage) "
+            "VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)",
+            (agg.day, agg.seconds_active, agg.seconds_idle,
+             agg.seconds_powered_off, agg.seconds_unknown,
+             agg.bytes_written_delta, agg.bytes_read_delta,
+             agg.sample_count, agg.coverage),
+        )
+    else:
+        conn.execute(
+            "UPDATE day_aggregates "
+            "SET active_seconds = ?, idle_seconds = ?, powered_off_seconds = ?, "
+            "    unknown_seconds = ?, bytes_written_delta = ?, bytes_read_delta = ?, "
+            "    sample_count = ?, coverage = ? "
+            "WHERE id = ?",
+            (agg.seconds_active, agg.seconds_idle,
+             agg.seconds_powered_off, agg.seconds_unknown,
+             agg.bytes_written_delta, agg.bytes_read_delta,
+             agg.sample_count, agg.coverage,
+             existing[0]),
+        )