Resolve an occurrence back to the rule behind it

The detail sheet looked its record up by the row's id. For an occurrence
that id carries a #date suffix and is not a key in the store, so `own` came
back undefined and the edit and delete buttons vanished on every recurring
row — and a save would have reached editEvent with an id it could not find
and quietly done nothing.

The suffix is deliberate: marks key off the occurrence so each time round
carries its own completion. There is still only one record to edit.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/shared/custom.ts

@@ -1,5 +1,5 @@
 import { z } from "zod";
-import { comesRoundEarly, Repeat } from "./recurrence.ts";
+import { comesRoundEarly, Repeat, ruleIdOf } from "./recurrence.ts";
 import type { Occurrence } from "./recurrence.ts";
 import { EventType, GachaEvent, Precision, slugify } from "./schema.ts";
 
@@ -271,3 +271,21 @@ export function asOccurrenceEvent(
     endPrecision: occurrence.endPrecision,
   };
 }
+
+/**
+ * The stored record a row belongs to, whichever kind of id it carries.
+ *
+ * A row may be one occurrence of a rule, whose id carries a `#` suffix and is
+ * deliberately not a key in the store. Marks, ignores and ticks key off that
+ * suffixed id — each time round has its own completion — but there is only ever
+ * one record to edit, and it is the rule.
+ *
+ * Total, and safe for a feed id: `ruleIdOf` returns anything without a
+ * separator unchanged, and a feed id is simply not in this store.
+ */
+export function recordFor(
+  events: CustomEvents,
+  rowId: string,
+): CustomEvent | undefined {
+  return events[ruleIdOf(rowId)];
+}