Ask only the recurrence rules, the live server settles it

A probe against the live 0.16.19 says a one-off event comes back from an
expanded query as id "eaaaaai" with baseEventId "i" — an instance id of
its own, and a base that is a different event. The clause that treated a
differing base as an occurrence of a series would therefore have gone on
calling every event recurring, which was the bug.

So recurrence rules alone decide it. What that gives up is an expanded
instance that arrives without its rules attached; whether Stalwart does
that is still to be checked against a real series.

web/src/store/__tests__/recurrence.test.ts

@@ -15,9 +15,9 @@ describe("isRecurring", () => {
   it("does not call a one-off event a series just because it has a baseEventId", () => {
     expect(isRecurring(ev({ baseEventId: "ev1" }))).toBe(false);
     expect(isRecurring(ev({}))).toBe(false);
-  });
-  it("still recognises an occurrence whose base is another event", () => {
-    expect(isRecurring(ev({ id: "ev1_2", baseEventId: "ev1" }))).toBe(true);
+    // The shape a live 0.16.19 returns for a one-off: an instance id of its own,
+    // and a base that is a different id. Neither makes it a series.
+    expect(isRecurring(ev({ id: "eaaaaai", baseEventId: "i" }))).toBe(false);
   });
   it("recognises a series by its recurrence rules", () => {
     expect(isRecurring(ev({ recurrenceRules: [{ "@type": "RecurrenceRule", frequency: "weekly" }] }))).toBe(true);