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/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', () => {