Follow Stalwart 0.16.21 in the mock

Four changes, each confirmed against a real 0.16.21 rather than read from
the changelog.

Synthetic recurrence ids are now built from an occurrence's recurrenceId
instead of its position, so they survive a write. This reverses a hazard
the mock reproduced on purpose: up to 0.16.20 writing one override
renumbered the series and a held id silently named a different date. A
five-week series was expanded live, its third occurrence retitled through
its synthetic id, and all five original ids re-read; every one still
resolved to its own date. The test that pinned the instability now pins
the stability, with two more around it.

Calendar/get and AddressBook/get return every property when properties is
omitted or null, shareWith included. Mailbox/get on the same server still
omits it, so that stripping stays and now applies to mailboxes alone.

EventSource ping events advertise the interval in seconds, not
milliseconds. The mock parses the parameter it used to ignore: a 30 s
floor, larger values honoured, 0 disables pings, a non-numeric value is a
400. The first ping now arrives one interval in rather than on connect,
which is what the server does.

CalendarEvent/set rejects create, update and destroy with forbidden when
the request asks for scheduling messages and the account may not send
them. MOCK_NO_SCHEDULING_SEND=1 develops against that account.

server/src/mock/recurrence.ts

@@ -1,5 +1,5 @@
 /**
- * Enough recurrence expansion for the mock to behave like Stalwart 0.16.20.
+ * Enough recurrence expansion for the mock to behave like Stalwart 0.16.21.
  *
  * The mock used to hand a recurring event back once, as its stored self. Three
  * things that only a live server showed were therefore impossible to develop
@@ -8,8 +8,8 @@
  * - an expanded query gives *everything* a synthetic id over a `baseEventId`,
  *   a one-off included, so `baseEventId` is no evidence of a series;
  * - an occurrence carries a `recurrenceId` and no rule of its own;
- * - 0.16.20 takes a write aimed at a synthetic id and turns it into a
- *   `recurrenceOverrides` entry rather than touching the series.
+ * - a write aimed at a synthetic id becomes a `recurrenceOverrides` entry
+ *   rather than touching the series.
  *
  * A mock that agrees with the client rather than with the server is how #26 and
  * #30 reached a live instance, so the refusals matter as much as the successes:
@@ -25,41 +25,41 @@ const MAX_ITERATIONS = 750;
 const DAYS = ["su", "mo", "tu", "we", "th", "fr", "sa"];
 
 /**
- * The id an occurrence is addressed by, which is only true until the next write.
+ * The id an occurrence is addressed by: its `recurrenceId`, not its position.
  *
  * Stalwart's are opaque; the mock's are parseable because it has to resolve
  * them, and nothing in ihasmail may read either.
  *
- * They are also deliberately **unstable**, because the real ones are.
- * **Confirmed live on 0.16.20 (2026-08-31):** a synthetic id encodes a position
- * in the expanded series, and writing a `recurrenceOverrides` entry adds a
- * component that renumbers it. A five-week series held `e i m q u` over
- * 03-01…03-29; after one override was written to 03-08 the same ids addressed
- * 03-01, 03-15, 03-29, 03-08, 03-22. Nothing was rejected — they just meant
- * different dates.
+ * **They are stable, and that is a change.** Up to 0.16.20 a synthetic id
+ * encoded a *position* in the expanded series, so writing one override
+ * renumbered the rest and a held id silently began addressing a different
+ * date — a hazard this file used to reproduce on purpose. 0.16.21 fixed it:
+ * an occurrence is now identified by its recurrence id.
  *
- * That is the hazard worth reproducing, and note which way round it goes: a
- * stale id is not *invalid*, it is *wrong*. A mock that expired them instead
- * would hand back a loud `notFound` and let a client that caches ids look
- * careful. So the numbering is shifted by the number of overrides — an
- * arbitrary stand-in for Stalwart's renumbering, with the one property that
- * matters: hold an id across a write and it silently addresses another date.
+ * **Confirmed live on 0.16.21 (2026-09-06):** a five-week weekly series was
+ * expanded, the third occurrence retitled through its synthetic id, and all
+ * five original ids re-read afterwards. Every one still resolved, and every
+ * one still named its own date; nothing was renumbered and nothing was
+ * `notFound`. Only the *order* of the ids from an expanded query changed —
+ * the overridden occurrence moved to the end of the list — which is why a
+ * client sorts by `start` rather than trusting query order.
+ *
+ * The real ids look nothing like these (`h1fo9uaaaaab` for the first of that
+ * series); what has to match is that holding one across a write stays correct.
  */
-export const syntheticId = (baseId: string, slot: number): string => `${baseId}-o${slot}`;
+const compact = (recurrenceId: string): string => recurrenceId.replace(/[-:]/g, "");
 
-export function parseSyntheticId(id: string): { baseId: string; slot: number } | null {
-  const m = /^(.+)-o(\d+)$/.exec(id);
-  return m ? { baseId: m[1]!, slot: Number(m[2]) } : null;
-}
+export const syntheticId = (baseId: string, recurrenceId: string): string =>
+  `${baseId}-r${compact(recurrenceId)}`;
 
-/** How far the id numbering has been rotated away from the series order. */
-function rotation(base: Obj): number {
-  return Object.keys((base.recurrenceOverrides as Record<string, Obj> | undefined) ?? {}).length;
-}
-
-/** The id slot this occurrence currently answers to. */
-export function slotOfOccurrence(base: Obj, occ: Occurrence): number {
-  return occ.index + rotation(base);
+export function parseSyntheticId(id: string): { baseId: string; recurrenceId: string } | null {
+  const m = /^(.+)-r(\d{8}T\d{6})$/.exec(id);
+  if (!m) return null;
+  const c = m[2]!;
+  const recurrenceId =
+    `${c.slice(0, 4)}-${c.slice(4, 6)}-${c.slice(6, 8)}` +
+    `T${c.slice(9, 11)}:${c.slice(11, 13)}:${c.slice(13, 15)}`;
+  return { baseId: m[1]!, recurrenceId };
 }
 
 /** `2026-08-31T09:00:00` — the naive local form the mock stores `start` in. */
@@ -104,8 +104,8 @@ export function expandOccurrences(base: Obj, from: Date, to: Date): Occurrence[]
   const emit = (index: number, at: Date): boolean => {
     const recurrenceId = localDateTime(at);
     const override = overrides[recurrenceId];
-    // An excluded date is simply gone from the expansion. Its slot is not
-    // reserved -- see `syntheticId` for why nothing here pretends otherwise.
+    // An excluded date is simply gone from the expansion. Nothing is
+    // reserved in its place, and no other occurrence's id moves because of it.
     if (override?.excluded === true) return true;
     /*
      * An override may move the occurrence, and then `start` and `recurrenceId`
@@ -115,9 +115,9 @@ export function expandOccurrences(base: Obj, from: Date, to: Date): Occurrence[]
      * came back `start: 2027-06-14T14:00:00` with `recurrenceId` still
      * `2027-06-14T09:00:00`.
      *
-     * Which is exactly why `recurrenceId` is what a client holds on to. It is
-     * the one name for this instance that neither a renumbering nor a move
-     * changes.
+     * Which is exactly why `recurrenceId` is what a client holds on to, and
+     * since 0.16.21 what the id is built from: the one name for this instance
+     * that a move does not change.
      */
     const start = (typeof override?.start === "string" ? override.start : null) ?? recurrenceId;
     const shown = parseLocal(start);
@@ -178,7 +178,7 @@ export function occurrenceView(base: Obj, occ: Occurrence): Obj {
   const view: Obj = { ...base };
   for (const k of SERIES_ONLY) delete view[k];
   Object.assign(view, occ.override ?? {});
-  view.id = syntheticId(base.id as string, slotOfOccurrence(base, occ));
+  view.id = syntheticId(base.id as string, occ.recurrenceId);
   view.baseEventId = base.id;
   view.start = occ.start;
   // Only a genuine instance of a series carries one. A one-off expanded into
@@ -229,10 +229,13 @@ export function splitOccurrencePatch(patch: Obj): { rejected?: string; applied:
   return { applied };
 }
 
-/** The occurrence a slot currently addresses — which is not a fixed thing. */
-export function occurrenceAt(base: Obj, slot: number): Occurrence | null {
-  const index = slot - rotation(base);
-  if (index < 0) return null;
+/**
+ * The occurrence a recurrence id addresses, which no later write moves.
+ *
+ * An id whose date the rule no longer generates — excluded, or past a `count`
+ * — resolves to nothing, and the caller turns that into `notFound`.
+ */
+export function occurrenceAt(base: Obj, recurrenceId: string): Occurrence | null {
   const all = expandOccurrences(base, new Date(-8640000000000), new Date(8640000000000));
-  return all.find((o) => o.index === index) ?? null;
+  return all.find((o) => o.recurrenceId === recurrenceId) ?? null;
 }