feat(#73): publish trustworthy first usage history

- 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

src/fenris/collector.py

@@ -16,6 +16,8 @@ from pathlib import Path
 from typing import Any, Dict, Optional, Tuple
 
 from .store import init_store, get_store_path
+from .monitoring_periods import ensure_period_open
+from .derive import find_previous_sample, derive_hours_from_interval
 
 
 class AcquisitionError(Exception):
@@ -221,7 +223,11 @@ def write_sample(
         open_segment(conn, now, identity, identity_key, identity_degraded)
         segment_opened = True
     
-    # Insert sample
+    # Get current segment_id for provenance
+    current_segment = find_current_segment(conn)
+    segment_id = current_segment["id"] if current_segment else None
+    
+    # Insert sample with segment_id
     cursor = conn.execute(
         """
         INSERT INTO samples (
@@ -229,8 +235,8 @@ def write_sample(
             percentage_used, available_spare, media_errors, power_on_hours,
             power_cycles, unsafe_shutdowns, temperature_c,
             data_units_written, data_units_read, bytes_written, bytes_read,
-            critical_warning
-        ) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
+            critical_warning, segment_id
+        ) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
         """,
         (
             sample["ts"],
@@ -252,6 +258,7 @@ def write_sample(
             sample["bytes_written"],
             sample["bytes_read"],
             sample["critical_warning"],
+            segment_id,
         ),
     )
     
@@ -262,6 +269,7 @@ def write_sample(
         "segment_reason": reason,
         "identity_key": identity_key,
         "identity_degraded": identity_degraded,
+        "segment_id": segment_id,
     }
 
 
@@ -303,11 +311,38 @@ def run_collection(
 
         
         try:
+            # Ensure monitoring period is open (issue #73 AC2)
+            ensure_period_open(conn, clock.utcnow())
+            
             # Validate invariants
             validate_sample_invariants(sample, conn)
             
-            # Write sample
-            write_sample(sample, identity, conn, clock)
+            # Write sample and get segment info
+            seg_info = write_sample(sample, identity, conn, clock)
+            
+            # Derive hour observations from interval with previous sample
+            try:
+                # 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
+                    current = {
+                        "id": current_id,
+                        "ts": sample["ts"],
+                        "bytes_written": sample["bytes_written"],
+                        "bytes_read": sample["bytes_read"],
+                        "power_on_hours": sample["power_on_hours"],
+                        "temperature_c": sample["temperature_c"],
+                        "data_units_written": sample["data_units_written"],
+                        "data_units_read": sample["data_units_read"],
+                    }
+                    derive_hours_from_interval(conn, prev, current)
+            except Exception:
+                # Derivation failure must not prevent sample persistence (issue #73 AC6)
+                pass
             
             return {
                 "ok": True,