fix(rollups): step one tier finer when the coarse tier comes back empty

The granularity router read only the coarsest tier the window cleared, and
a coarse bucket that starts before the window is left out. Data that sat
only in that leading partial bucket, such as three months at the start of
a 731-day window, read as no data at all. The router now steps one tier
finer until a tier has buckets, down to DAY.

src/lib/rollups/__tests__/measurement-read-wmy.test.ts

@@ -213,7 +213,33 @@ describe("readBestGranularityRollups", () => {
   it("returns null when the window holds no buckets", async () => {
     const result = await readBestGranularityRollups("user", "WEIGHT", 1095);
     expect(result).toBeNull();
-    expect(queryRaw).toHaveBeenCalledTimes(1);
+    // YEAR, MONTH and WEEK folds, then the DAY tier.
+    expect(queryRaw).toHaveBeenCalledTimes(3);
+    expect(findMany).toHaveBeenCalledTimes(1);
+  });
+
+  it("steps one tier finer when the data sits only in the leading partial bucket", async () => {
+    // A 731-day window on 2026-09-26 starts 2024-09-25. October to December
+    // 2024 fold into the YEAR bucket 2024-01-01, which starts before the
+    // window and is left out; the MONTH tier starts inside it.
+    vi.setSystemTime(new Date("2026-09-26T12:00:00.000Z"));
+    const readings: Array<[string, number]> = [
+      ["2024-10-05", 80],
+      ["2024-11-05", 81],
+      ["2024-12-05", 82],
+    ];
+    queryRaw
+      .mockResolvedValueOnce([folded("2024-01-01T00:00:00.000Z", readings)])
+      .mockResolvedValueOnce(
+        readings.map(([day, value]) =>
+          folded(`${day.slice(0, 7)}-01T00:00:00.000Z`, [[day, value]]),
+        ),
+      );
+    const result = await readBestGranularityRollups("user", "WEIGHT", 731);
+    expect(result?.granularity).toBe("MONTH");
+    expect(result?.rows.map((r) => r.mean)).toEqual([80, 81, 82]);
+    expect(sqlOf(queryRaw.mock.calls[0])).toContain("'year'");
+    expect(sqlOf(queryRaw.mock.calls[1])).toContain("'month'");
   });
 
   it("composes a coarse bucket's mean, spread and slope from its days", async () => {

src/lib/rollups/measurement-read-wmy.ts

@@ -125,25 +125,32 @@ export async function readBestGranularityRollups(
     userPriorityJson !== undefined
       ? userPriorityJson
       : await loadUserSourcePriority(userId);
-  // Every tier is folded from the same canonical DAY buckets, so a tier
-  // either has coverage or none has: pick the coarsest floor the window
-  // clears and read it once.
-  const floor = GRANULARITY_FLOORS.find((f) => windowDays >= f.minWindowDays);
-  if (!floor) return null;
-  const rows = await readGranularity(
-    userId,
-    type,
-    floor.granularity,
-    since,
-    // Trailing-window semantics: no upper bound. This router serves the
-    // "last N days to now" probes (summaries-slice / health-score); the
-    // requested-window bounding lives on `readTieredRollupSeries`.
-    null,
-    priority,
+  // Every tier is folded from the same canonical DAY buckets, but a coarse
+  // bucket that starts before `since` is left out (it holds only part of its
+  // days). Data that sits only in that leading partial bucket, say three
+  // months at the start of a 731-day window, reads as empty at YEAR: step one
+  // tier finer until a tier has buckets, down to DAY, which starts inside the
+  // window by construction.
+  const floors = GRANULARITY_FLOORS.filter(
+    (f) => windowDays >= f.minWindowDays,
   );
-  return rows && rows.length > 0
-    ? { granularity: floor.granularity, rows }
-    : null;
+  for (const floor of floors) {
+    const rows = await readGranularity(
+      userId,
+      type,
+      floor.granularity,
+      since,
+      // Trailing-window semantics: no upper bound. This router serves the
+      // "last N days to now" probes (summaries-slice / health-score); the
+      // requested-window bounding lives on `readTieredRollupSeries`.
+      null,
+      priority,
+    );
+    if (rows && rows.length > 0) {
+      return { granularity: floor.granularity, rows };
+    }
+  }
+  return null;
 }
 
 /**