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);

web/src/store/calendar.ts

@@ -300,15 +300,15 @@ export const useCalendar = create<CalendarState>((set, get) => ({
 /**
  * Whether an event is part of a series.
  *
- * Not the same question as "does it have a baseEventId": `CalendarEvent/query`
- * runs with `expandRecurrences`, and Stalwart sets `baseEventId` on every event
- * it returns that way — a one-off event included, pointing at itself. Recurrence
- * rules are what make a series, so those are the question; a base that is some
- * *other* event means this is one occurrence of one, whether or not the expanded
- * instance carried the rules along with it.
+ * Not the same question as "does it have a baseEventId", nor "is that base some
+ * other event". `CalendarEvent/query` runs with `expandRecurrences`, and Stalwart
+ * hands back an instance id for everything it returns that way — a one-off event
+ * included, whose own id (`eaaaaai`) differs from its base (`i`), verified
+ * against a live 0.16.19. Recurrence rules are what make a series, so those are
+ * what we ask about.
  */
 export function isRecurring(ev: CalendarEvent): boolean {
-  return Boolean(ev.recurrenceRules?.length || ev.excludedRecurrenceRules?.length || (ev.baseEventId && ev.baseEventId !== ev.id));
+  return Boolean(ev.recurrenceRules?.length || ev.excludedRecurrenceRules?.length);
 }
 
 export function toInstance(e: CalendarEvent, calendars: Record<Id, Calendar>): EventInstance | null {