store: return one calendar row per event

CalendarEvents range-scanned the key prefix and returned every historical
row, voided ones included. The facts table is append-only and the event
key is day plus summary, so moving a standup from 14:00 to 16:00 left two
rows under one key. The day plan prints a time per line, so it recited
both and told the owner he had two standups.

The query now drops voided rows, keeps the latest row within a source,
and prefers the best-evidenced source across them, so a notification
relay guessing at a meeting cannot displace the calendar read of it.
Found in review of #58.

internal/store/facts.go

@@ -90,6 +90,13 @@ func (s *Store) RecentFacts(ctx context.Context, n int) ([]Fact, error) {
 // the same answer. The source stays on each Fact, along with its confidence, so
 // the caller can hedge a reading it did not get from a calendar server —
 // filtering by source here would have thrown that judgement away.
+//
+// One row per event, not one per write. The facts table is append-only, so a
+// standup moved from 14:00 to 16:00 leaves two rows under the same key, and the
+// day plan used to recite both as if the owner had two meetings. Voided rows
+// are excluded, the latest row wins within a source, and the best-evidenced
+// source wins across them — a calendar read beats the notification relay that
+// guessed at the same meeting.
 func (s *Store) CalendarEvents(ctx context.Context, from, to time.Time) ([]Fact, error) {
 	prefixFrom := calendar.KeyPrefixForDay(from)
 	prefixTo := calendar.KeyPrefixForDay(to)
@@ -104,7 +111,8 @@ func (s *Store) CalendarEvents(ctx context.Context, from, to time.Time) ([]Fact,
           FROM facts
          WHERE source IN (`+placeholders(len(sources))+`)
            AND key >= ? AND key < ?
-         ORDER BY key`, args...)
+           AND id NOT IN (SELECT voids_id FROM facts WHERE voids_id IS NOT NULL)
+         ORDER BY key, ts, id`, args...)
 	if err != nil {
 		return nil, fmt.Errorf("calendar events: %w", err)
 	}
@@ -117,7 +125,46 @@ func (s *Store) CalendarEvents(ctx context.Context, from, to time.Time) ([]Fact,
 		}
 		out = append(out, f)
 	}
-	return out, rows.Err()
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return latestPerCalendarKey(out), nil
+}
+
+// latestPerCalendarKey reduces the append-only rows for one day to one row per
+// event key. Input must be ordered by key then oldest-first, so the last row
+// seen for a key and source is that source's current value.
+func latestPerCalendarKey(in []Fact) []Fact {
+	type slot struct {
+		bySource map[string]Fact
+		order    []string
+	}
+	var keys []string
+	byKey := map[string]*slot{}
+	for _, f := range in {
+		s, ok := byKey[f.Key]
+		if !ok {
+			s = &slot{bySource: map[string]Fact{}}
+			byKey[f.Key] = s
+			keys = append(keys, f.Key)
+		}
+		if _, seen := s.bySource[f.Source]; !seen {
+			s.order = append(s.order, f.Source)
+		}
+		s.bySource[f.Source] = f
+	}
+	out := make([]Fact, 0, len(keys))
+	for _, k := range keys {
+		s := byKey[k]
+		best := s.bySource[s.order[0]]
+		for _, src := range s.order[1:] {
+			if s.bySource[src].Confidence > best.Confidence {
+				best = s.bySource[src]
+			}
+		}
+		out = append(out, best)
+	}
+	return out
 }
 
 // LatestFactBySource — provenance-scoped. A rule on `service_down` trusts only