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/collector.py

@@ -325,7 +325,7 @@ def run_collection(
                 # Find the sample we just wrote
                 cursor = conn.execute("SELECT id FROM samples ORDER BY id DESC LIMIT 1")
                 current_id = cursor.fetchone()[0]
-                
+
                 prev = find_previous_sample(conn, seg_info.get("segment_id"), current_id)
                 if prev is not None:
                     # Build current sample dict for derivation
@@ -340,6 +340,12 @@ def run_collection(
                         "data_units_read": sample["data_units_read"],
                     }
                     derive_hours_from_interval(conn, prev, current)
+
+                # Rebuild day aggregates from hour observations
+                from .day_aggregate import derive_all_days, persist_day_aggregate
+                for agg in derive_all_days(conn):
+                    persist_day_aggregate(conn, agg)
+                conn.commit()
             except Exception:
                 # Derivation failure must not prevent sample persistence (issue #73 AC6)
                 pass