feat(report): Stage-0 usage gate counts from a baseline date (default 2026-06-11)

The return-usage gate (ADR-016) counted distinct active weeks over ALL history,
so pre-launch noise — testing churn and the period the pilot sat blocked on zero
summaries — would inflate the signal. Add a baseline: ActiveWeeks(ctx, since)
filters login_events + summary_actions to seen_at/acted_at >= since. The report
command sets it to TAPIR_USAGE_GATE_START (YYYY-MM-DD, default 2026-06-11 — the
morning the pilot was unblocked) and prints the baseline. The gate now measures
whether users RETURN once it genuinely works.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/adapters/store/report_test.go

@@ -3,6 +3,7 @@ package store_test
 import (
 	"context"
 	"testing"
+	"time"
 
 	"github.com/jackc/pgx/v5/pgxpool"
 	"github.com/stretchr/testify/require"
@@ -54,7 +55,7 @@ func TestActiveWeeksCountsDistinctWeeksAcrossReadsAndActs(t *testing.T) {
 		   ($1, 'vid-2', 'saved',   '2026-01-19T18:00:00Z')`, userA)
 	require.NoError(t, err)
 
-	got, err := s.ActiveWeeks(ctx)
+	got, err := s.ActiveWeeks(ctx, time.Time{}) // zero since = no lower bound
 	require.NoError(t, err)
 	require.Len(t, got, 2, "both identity users must appear")
 
@@ -72,7 +73,33 @@ func TestActiveWeeksEmptyWhenNoUsers(t *testing.T) {
 	s := newStore(t)
 	resetDB(t, rawPool(t))
 
-	got, err := s.ActiveWeeks(ctx)
+	got, err := s.ActiveWeeks(ctx, time.Time{})
 	require.NoError(t, err)
 	require.Empty(t, got)
 }
+
+// TestActiveWeeksExcludesBeforeGateStart proves the baseline cutoff: activity
+// before `since` does not count, so pre-launch noise (testing, the pilot's blocked
+// period) is excluded from the Stage-0 return-usage gate (ADR-016).
+func TestActiveWeeksExcludesBeforeGateStart(t *testing.T) {
+	ctx := context.Background()
+	s := newStore(t)
+	p := rawPool(t)
+	resetDB(t, p)
+
+	seedReportUser(t, p, userA, "subject-a", "Ada")
+
+	// One read well before the baseline, two reads in distinct weeks after it.
+	_, err := p.Exec(ctx,
+		`INSERT INTO login_events (user_id, seen_at) VALUES
+		   ($1, '2026-05-01T09:00:00Z'),
+		   ($1, '2026-06-12T09:00:00Z'),
+		   ($1, '2026-06-19T09:00:00Z')`, userA)
+	require.NoError(t, err)
+
+	since := time.Date(2026, 6, 11, 0, 0, 0, 0, time.UTC)
+	got, err := s.ActiveWeeks(ctx, since)
+	require.NoError(t, err)
+	require.Len(t, got, 1)
+	require.Equal(t, 2, got[0].ActiveWeeks, "only the two post-baseline weeks count; the May read is excluded")
+}