feat(timeline): render a same-year curated event as its cluster card header

A curated event with letters in its own band now becomes the contained card header
(glyph, title, date, provenance, edit pencil) instead of a separate floating pill —
the title reads once. Derived life-events, world-bands, and letterless event pills
are unchanged (REQ-001 amended for curated-with-letters; the identity fixture now
links its letter to the curated event so the letterless world band stays a band).

Refs #827

frontend/src/lib/timeline/grouping-event-layer-identity.svelte.spec.ts

@@ -46,13 +46,18 @@ function eventLayerSignature(): string {
 	});
 }
 
+// Brief A links to the curated event p1 (Hochzeit), not the world band — so the world band
+// stays letterless and renders as a plain band in every mode (REQ-001). Under the #827 redesign
+// a curated event WITH letters becomes its cluster card's header, so the signature tracks the
+// stable layer: the letterless world band's marker count and the two titles, which all survive
+// regardless of whether Hochzeit renders as a pill (Datum) or a card header (grouped).
 const mixed = () =>
 	makeTimelineDTO({
 		years: [
 			makeYear(1915, [
 				worldBand('Erster Weltkrieg'),
 				eventPill('Hochzeit'),
-				makeEntry({ documentId: 'a', title: 'Brief A', linkedEventId: 'h1' }),
+				makeEntry({ documentId: 'a', title: 'Brief A', linkedEventId: 'p1' }),
 				makeEntry({
 					documentId: 'b',
 					title: 'Brief B',