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.

scripts/backfill-rollups.ts

@@ -19,7 +19,7 @@ import { prisma } from "@/lib/db";
 import {
   ALL_GRANULARITIES,
   recomputeUserRollups,
-} from "@/lib/measurements/rollups";
+} from "@/lib/rollups/measurement-rollups";
 
 interface CliOptions {
   userId: string | null;