fix(timeline): keep HISTORICAL events out of inline clustering

buildEventLookup keyed on `kind === 'EVENT' && eventId` with no type
check, so a HISTORICAL curated event with ≥1 linked letter entered the
lookup and rendered as a mint EventCluster card — silently downgrading
from the full-width WorldBand that #779 REQ-009 mandates ("world-bands
render exactly as before").

The lookup now excludes `type === 'HISTORICAL'`, so a world event always
keeps its WorldBand and its letters stay loose chronological. Closes the
spec gap pinned as REQ-014. Fixes review finding #2.

Refs #850
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/timeline/TimelineView.svelte.spec.ts

@@ -371,4 +371,37 @@ describe('TimelineView', () => {
 		const titles = (document.body.textContent?.match(/Ein gewaltiger Stadtbrand/g) ?? []).length;
 		expect(titles).toBe(1);
 	});
+
+	it('keeps a HISTORICAL event a WorldBand with a same-year linked letter — never clusters (REQ-014)', () => {
+		const evId = 'dddddddd-dddd-dddd-dddd-dddddddddddd';
+		const world = makeEntry({
+			kind: 'EVENT',
+			type: 'HISTORICAL',
+			derived: false,
+			eventId: evId,
+			eventDate: '1916-07-01',
+			precision: 'DAY',
+			title: 'Schlacht an der Somme',
+			senderName: '',
+			receiverName: '',
+			documentId: undefined
+		});
+		const letter = makeEntry({
+			eventDate: '1916-05-10',
+			documentId: 'doc-world-linked',
+			title: 'Brief von der Front',
+			linkedEventId: evId
+		});
+		render(TimelineView, {
+			timeline: makeTimelineDTO({ years: [makeYear(1916, [world, letter])] })
+		});
+		// the world event stays a full-width band — no contained event card
+		expect(document.querySelector('[data-testid="event-card"]')).toBeNull();
+		expect(document.querySelector('a.lcard.ev')).toBeNull();
+		// the linked letter renders loose on the spine, not inside a card
+		expect(document.querySelector('.letter-row')).not.toBeNull();
+		// and the band keeps its WorldBand "· historisch" register
+		expect(document.body.textContent).toContain(m.timeline_layer_historical_suffix());
+		expect(document.body.textContent).toContain('Schlacht an der Somme');
+	});
 });

frontend/src/lib/timeline/eventClustering.spec.ts

@@ -44,6 +44,18 @@ describe('eventClustering — buildEventLookup', () => {
 		};
 		expect(buildEventLookup(timeline).size).toBe(0);
 	});
+
+	it('excludes a HISTORICAL event so its letters stay loose, keeping its WorldBand (REQ-014)', () => {
+		const timeline: TimelineDTO = {
+			years: [
+				{ year: 1916, entries: [makeEvent({ eventId: EV_A, type: 'HISTORICAL', title: 'Somme' })] }
+			],
+			undated: []
+		};
+		const lookup = buildEventLookup(timeline);
+		expect(lookup.has(EV_A)).toBe(false);
+		expect(lookup.size).toBe(0);
+	});
 });
 
 describe('eventClustering — splitYearLetters', () => {

frontend/src/lib/timeline/eventClustering.ts

@@ -25,14 +25,18 @@ export interface SplitLetters {
  * Maps each curated event present in the (already layer-filtered) timeline to its title. These
  * are the only events a letter may cluster under — a letter whose `linkedEventId` is absent here
  * links to an event the #780 layer filter removed, so it falls back to a loose chronological
- * letter (filter-then-cluster, REQ-008). Curated events carry an `eventId`; derived life-events
- * and letters do not, so they never enter the lookup.
+ * letter (filter-then-cluster, REQ-008). Curated PERSONAL events carry an `eventId`; derived
+ * life-events and letters do not, so they never enter the lookup. HISTORICAL events are excluded
+ * too: a world event always keeps its full-width WorldBand and never clusters, even with linked
+ * letters (REQ-014) — those letters stay loose.
  */
 export function buildEventLookup(timeline: TimelineDTO): Map<string, string> {
 	const lookup = new Map<string, string>();
 	const collect = (entries: TimelineEntryDTO[]) => {
 		for (const entry of entries) {
-			if (entry.kind === 'EVENT' && entry.eventId) lookup.set(entry.eventId, entry.title ?? '');
+			if (entry.kind === 'EVENT' && entry.eventId && entry.type !== 'HISTORICAL') {
+				lookup.set(entry.eventId, entry.title ?? '');
+			}
 		}
 	};
 	for (const band of timeline.years) collect(band.entries);