fix(sleep): restore the last-night hypnogram timeline bar

The staged bar is gated on the night carrying more than one distinct stage
boundary. The guard read the segment END instants, so an ordinary multi-stage
night whose segments share a right edge — the shape a summary-derived writer
reconstructs to — collapsed to the breakdown footer with no bar. Gate on the
distinct START instants instead, so a night with timed stages keeps its
timeline and only a session recorded on a single instant falls back to the
breakdown. Covers the chart-vs-breakdown branch the test suite did not.

src/components/insights/__tests__/sleep-hypnogram.test.tsx

@@ -92,3 +92,54 @@ describe("<SleepHypnogram> — lane derivation", () => {
     expect(src).toContain("ticks={lanes.map((_, i) => lanes.length - 1 - i)}");
   });
 });
+
+describe("<SleepHypnogram> — timeline bar visibility", () => {
+  const sharedEnd = "2026-06-09T06:00:00.000Z";
+
+  it("draws the timeline bar for a multi-stage night whose segments share a right edge", () => {
+    // Distinct STARTS but a shared END instant — the shape a summary-derived
+    // writer reconstructs to. The earlier end-instant guard hid the bar here;
+    // gating on the start instant restores it. (Regression: bar vanished,
+    // leaving only the breakdown numbers.)
+    const session: SleepHypnogramSession = {
+      ...baseSession,
+      segments: [
+        {
+          stage: "CORE",
+          start: "2026-06-08T22:30:00.000Z",
+          end: sharedEnd,
+          minutes: 200,
+        },
+        {
+          stage: "DEEP",
+          start: "2026-06-09T01:00:00.000Z",
+          end: sharedEnd,
+          minutes: 100,
+        },
+        {
+          stage: "REM",
+          start: "2026-06-09T03:00:00.000Z",
+          end: sharedEnd,
+          minutes: 120,
+        },
+      ],
+    };
+    expect(render(session)).toContain('role="img"');
+  });
+
+  it("collapses to the breakdown footer for a degenerate single-instant session", () => {
+    // Every segment stamped on ONE instant — no real timeline, so only the
+    // breakdown footer, no bar.
+    const instant = "2026-06-09T06:00:00.000Z";
+    const session: SleepHypnogramSession = {
+      ...baseSession,
+      segments: [
+        { stage: "CORE", start: instant, end: instant, minutes: 200 },
+        { stage: "DEEP", start: instant, end: instant, minutes: 100 },
+      ],
+    };
+    const html = render(session);
+    expect(html).not.toContain('role="img"');
+    expect(html).toContain('data-slot="sleep-hypnogram-breakdown"');
+  });
+});

src/components/insights/sleep-hypnogram.tsx

@@ -209,15 +209,17 @@ export function SleepHypnogram({ session }: SleepHypnogramProps) {
   // need any positive per-stage minutes. A legacy stage-summary night (no
   // segment spans) still shows its breakdown rather than collapsing the
   // whole card. "Unavailable" is reserved for a truly empty session.
-  // Spans must also carry more than one distinct end instant: a summary-
-  // shaped writer (per-stage night totals all stamped on the sleep end)
-  // reconstructs into bars that all touch the night's right edge — that
-  // is a breakdown wearing a timeline costume, so paint it as the
-  // breakdown footer instead.
+  // Spans must also carry more than one distinct START instant: a real
+  // night with timed stages steps through the clock, so its segments begin
+  // at different instants and earn the timeline bar. Only a degenerate
+  // session whose every segment starts on a single instant collapses to the
+  // breakdown footer. (Gating on the END instant instead hid the bar for an
+  // ordinary multi-stage night whose segments happened to share a right
+  // edge — the regression this restores.)
   const hasTrack =
     spans.length > 0 &&
     (spans.length === 1 ||
-      new Set(spans.map((s) => s.x2)).size > 1);
+      new Set(spans.map((s) => s.x1)).size > 1);
   const hasBreakdown = breakdown.length > 0;
 
   return (