fix(coach): name a night in the prose by the key the sleep table uses

The Coach's recent nights carried the wake instant in UTC as their date,
while the sleep table under the answer keys a night by the local day it
ends on. East of UTC the two named different days for the same night.
The snapshot now gives the night's own key.

src/lib/ai/coach/snapshot-blocks/__tests__/sleep-block-night-key.test.ts

@@ -0,0 +1,44 @@
+/**
+ * v1.39.4 — the Coach names a night by the same key as the sleep table
+ * under its answer: the local day the night ends on.
+ */
+import { describe, expect, it, vi } from "vitest";
+
+vi.mock("@/lib/logging/context", () => ({ annotate: vi.fn() }));
+
+import { reconstructSleepNights } from "@/lib/analytics/sleep-night";
+import { buildSleepTimelineBlock } from "../sleep-block";
+
+const TZ = "Pacific/Auckland";
+
+describe("buildSleepTimelineBlock — night keys", () => {
+  it("keys a night east of UTC by its local wake day, as the table does", () => {
+    // Woke at 07:00 on 27 September in Auckland: still the 26th in UTC.
+    const rows = [
+      {
+        value: 450,
+        measuredAt: new Date("2026-09-26T19:00:00.000Z"),
+        sleepStage: null,
+        source: "APPLE_HEALTH" as const,
+      },
+    ];
+    const snapshot: Record<string, unknown> = {};
+    buildSleepTimelineBlock({
+      sleepRows: rows,
+      sourcePriorityJson: null,
+      userTz: TZ,
+      recentCutoff: new Date("2026-09-01T00:00:00.000Z"),
+      snapshot,
+      metrics: new Set(),
+      counts: {},
+      registerBlock: () => {},
+      groundingValues: new Map(),
+    });
+    const recent = (
+      snapshot.sleep as { timeline: { recent: Array<{ date: string }> } }
+    ).timeline.recent;
+    const tableKey = reconstructSleepNights(rows, TZ, null)[0].night;
+    expect(tableKey).toBe("2026-09-27");
+    expect(recent[0].date).toBe(tableKey);
+  });
+});

src/lib/ai/coach/snapshot-blocks/sleep-block.ts

@@ -97,7 +97,11 @@ export function buildSleepTimelineBlock(
       .filter((n) => n.measuredAt >= recentCutoff)
       .map((n) => {
         const row: Record<string, unknown> = {
-          date: n.measuredAt,
+          // The night's own key, the local day it ends on: the key the
+          // sleep table under the answer and every sleep surface use. The
+          // wake instant in UTC named the day before east of UTC, so the
+          // prose and the table disagreed about which night it was.
+          date: n.night,
           weekday: tzWeekday(n.measuredAt, userTz),
           minutes: Math.round(n.asleepMinutes),
         };