Calendar: never mutate an occurrence by an id we are holding

Verified against the live 0.16.20 instance, which found two things the
mock had guessed wrong about.

A synthetic id encodes a position in the expanded series, and writing a
`recurrenceOverrides` entry renumbers it. A five-week series came back as
`e i m q u` over 03-01..03-29; after one override was written to 03-08
the same five ids addressed 03-01, 03-15, 03-29, 03-08 and 03-22. Nothing
was rejected. A stale id is not invalid, it is wrong - a confident answer
about the wrong day - so a delete meant for one occurrence removes
another.

`recurrenceId` is the stable name for a slot in a series, because it is
the date. `updateEvent` and `destroyEvent` now look the current id up by
it immediately before acting, and refuse outright when the date has left
the series rather than falling back to the id in hand.

The mock had this exactly backwards: it kept ids stable on purpose, which
agreed with the belief that is wrong. It now renumbers too - a different
permutation to Stalwart's, with the property that matters - and a test
holds an id across a write and watches it change meaning.

Second finding: the inherited properties are dropped *after* the server
has decided to write an override, so a patch made only of them still
writes one, carrying the server-filled start and duration and nothing
else. `{"privacy":"private"}` on one occurrence answered "updated", left
privacy untouched, and left that date with no title at all. Sending
nothing when narrowing empties a patch was written as a principle - a
request whose response could only be a meaningless "updated" is worse
than no request - and it turns out to prevent real data loss.

Both recorded in KNOWN-ISSUES with the dates they were confirmed on.

server/src/mock/index.ts

@@ -5,7 +5,7 @@
  */
 import { createServer, type IncomingMessage, type ServerResponse } from "node:http";
 import { randomUUID } from "node:crypto";
-import { expandOccurrences, occurrenceAt, occurrenceView, parseSyntheticId, splitOccurrencePatch, syntheticId, type Occurrence } from "./recurrence.js";
+import { expandOccurrences, occurrenceAt, occurrenceView, parseSyntheticId, slotOfOccurrence, splitOccurrencePatch, syntheticId, type Occurrence } from "./recurrence.js";
 import { parseOtpauthUrl, verifyTotp } from "../totp.js";
 import { holdUntilOf, undoStatusOf } from "./futurerelease.js";
 
@@ -414,7 +414,7 @@ function resolveEvent(list: Obj[], id: string): { base: Obj; occ?: Occurrence }
   if (!parsed) return null;
   const base = list.find((x) => x.id === parsed.baseId);
   if (!base) return null;
-  const occ = occurrenceAt(base, parsed.index);
+  const occ = occurrenceAt(base, parsed.slot);
   return occ ? { base, occ } : null;
 }
 
@@ -929,7 +929,7 @@ const handlers: Record<string, Handler> = {
     const from = filter.after ? new Date(filter.after as string) : new Date(-8640000000000);
     const to = filter.before ? new Date(filter.before as string) : new Date(8640000000000);
     const ids: string[] = [];
-    for (const e of matching) for (const occ of expandOccurrences(e, from, to)) ids.push(syntheticId(e.id as string, occ.index));
+    for (const e of matching) for (const occ of expandOccurrences(e, from, to)) ids.push(syntheticId(e.id as string, slotOfOccurrence(e, occ)));
     return { accountId: a.accountId ?? ACCOUNT, queryState: "1", canCalculateChanges: false, position: 0, ids, total: ids.length };
   },
   "CalendarEvent/get": (a) => {