refactor(rollups): move measurement-rollups under src/lib/rollups umbrella

Creates the new src/lib/rollups/ aggregator directory and relocates
the main measurement rollup module:

- src/lib/measurements/rollups.ts → src/lib/rollups/measurement-rollups.ts
- src/lib/measurements/__tests__/rollups.test.ts →
  src/lib/rollups/__tests__/measurement-rollups.test.ts

Every importer of @/lib/measurements/rollups is rewritten to
@/lib/rollups/measurement-rollups. The orphan sibling reference inside
src/lib/measurements/rollup-read.ts (still in its old home until the
next batch) is promoted to the @/lib alias so the chain keeps
typechecking through the in-flight move.

Pure relocation — no behaviour change, no exported-surface change. The
umbrella exists so the rollup contract is discoverable from a single
directory before the iOS sprint locks the boundary.

src/app/api/import/__tests__/route.test.ts

@@ -31,10 +31,10 @@ vi.mock("@/lib/logging/context", () => ({
 // assert that the import path now folds the persistent rollup tier
 // for each touched (type, day). Mood rollup mock stays no-op because
 // the existing v1.4.39 W-MOOD hook already covers the moodlog branch.
-vi.mock("@/lib/measurements/rollups", async () => {
+vi.mock("@/lib/rollups/measurement-rollups", async () => {
   const actual = await vi.importActual<
-    typeof import("@/lib/measurements/rollups")
-  >("@/lib/measurements/rollups");
+    typeof import("@/lib/rollups/measurement-rollups")
+  >("@/lib/rollups/measurement-rollups");
   return {
     ...actual,
     recomputeBucketsForMeasurement: vi.fn().mockResolvedValue(undefined),
@@ -49,7 +49,7 @@ import { NextRequest } from "next/server";
 import { POST } from "../route";
 import { checkRateLimit } from "@/lib/rate-limit";
 import { prisma } from "@/lib/db";
-import { recomputeBucketsForMeasurement } from "@/lib/measurements/rollups";
+import { recomputeBucketsForMeasurement } from "@/lib/rollups/measurement-rollups";
 
 beforeEach(() => {
   vi.resetAllMocks();

src/app/api/import/route.ts

@@ -21,7 +21,7 @@ import { recomputeUserMoodRollups } from "@/lib/mood/rollups";
 import {
   collapseToTypeDayKeys,
   recomputeBucketsForMeasurement,
-} from "@/lib/measurements/rollups";
+} from "@/lib/rollups/measurement-rollups";
 import type { MeasurementType } from "@/generated/prisma/client";
 
 // Derived from canonical enum so round-trip export → import covers every