Key timeline weeks by ISO week, not the Monday date

periodOf keyed a week by the Monday's YYYY-MM-DD, which collides with
the day-key form. Weeks now key as YYYY-Www (ISO 8601, e.g. 2025-W32);
the week-year comes from the week's Thursday, so it stays correct across
the January/December boundary. The Monday-start boundary and startsAt
are unchanged. Week tests now assert the ISO form.

Model: opus-4-8

test/library/read.test.ts

@@ -287,8 +287,9 @@ describe("lib.timeline grouping", () => {
             ],
         );
         const groups = apis(records).timeline.groups({ groupBy: "week" });
-        expect(groups.map((g) => g.key)).toEqual(["2024-01-22", "2024-01-15"]);
-        const first = groups.find((g) => g.key === "2024-01-15")!;
+        // ISO week keys: 2024-01-15 is in 2024-W03, the next Monday in 2024-W04.
+        expect(groups.map((g) => g.key)).toEqual(["2024-W04", "2024-W03"]);
+        const first = groups.find((g) => g.key === "2024-W03")!;
         expect(first.startsAt).toBe(Date.UTC(2024, 0, 15));
         // The Sunday belongs to the Monday-started week, not the next one.
         expect(first.fileIDs.sort((a, b) => a - b)).toEqual([1, 2, 3]);
@@ -301,7 +302,7 @@ describe("lib.timeline grouping", () => {
             [file(1, 1, { creationTime: micros(Date.UTC(2024, 0, 14, 12)) })],
         );
         const groups = apis(records).timeline.groups({ groupBy: "week" });
-        expect(groups.map((g) => g.key)).toEqual(["2024-01-08"]);
+        expect(groups.map((g) => g.key)).toEqual(["2024-W02"]);
         expect(groups[0]!.startsAt).toBe(Date.UTC(2024, 0, 8));
     });