feat(events): publish runId on a public calendar run entry (Rust D125)

A run entry on GET /public/events now names its run, the same id the
event page already publishes on each occurrence and `?run=` takes. A
Rust map marker carries core's run id and nothing else about its event,
so without this the app could only find the event by fetching every
event page.

A projected entry has no runId: nothing is committed to it. Rehearsals
and unlisted events stay absent from the calendar, so their markers
stay unlinked. The web calendar ignores the field.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E14m6SuuY6i1vASFeGDBeY

server/src/model/events/eventPublic.model.js

@@ -103,9 +103,18 @@ function phaseLabel(spec, phaseId) {
   return (phase && (phase.label || phase.id)) || null
 }
 
-/** One calendar entry, from a materialised run. */
+/**
+ * One calendar entry, from a materialised run.
+ *
+ * **`runId` is published because the event page already publishes it** on every
+ * occurrence, and `?run=` takes it. The calendar was the one public shape that
+ * named a run without saying which, so a client holding a run id from elsewhere
+ * (a module's map marker) had no way to find its event but to fetch every event
+ * page. A projection has none: nothing is committed to it.
+ */
 const publicRunEntry = (run) => ({
   kind: 'run',
+  runId: run.id,
   title: run.definition_title,
   slug: run.definition_slug,
   seriesName: run.series_name || null,