feat(timeline): add WorldBand for HISTORICAL context bands

Full-width muted band; RANGE renders a span pill (1914–1918) with a Zeitraum
aria-label (REQ-009); a RANGE with no end degrades to the start year, no pill,
no crash (REQ-010). World glyph is a redundant non-color cue with sr-only label
(REQ-018); text uses text-ink-2 to hold AA in both themes (REQ-019).

Refs #779
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -0,0 +1,48 @@
+import { describe, it, expect, afterEach } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import WorldBand from './WorldBand.svelte';
+import { makeEntry } from './test-factories';
+
+afterEach(() => cleanup());
+
+function historical(overrides = {}) {
+	return makeEntry({
+		kind: 'EVENT',
+		derived: false,
+		type: 'HISTORICAL',
+		title: 'Erster Weltkrieg',
+		senderName: '',
+		receiverName: '',
+		precision: 'RANGE',
+		eventDate: '1914-01-01',
+		eventDateEnd: '1918-12-31',
+		documentId: undefined,
+		...overrides
+	});
+}
+
+describe('WorldBand', () => {
+	it('renders the historical title with the world glyph + "Weltgeschehen" cue (REQ-018)', () => {
+		render(WorldBand, { entry: historical() });
+		expect(document.body.textContent).toContain('Erster Weltkrieg');
+		const hidden = document.querySelector('[aria-hidden="true"]');
+		expect(hidden?.textContent).toBe('◍');
+		const srOnly = document.querySelector('.sr-only');
+		expect(srOnly?.textContent).toBe('Weltgeschehen');
+	});
+
+	it('renders a RANGE span pill 1914–1918 with a Zeitraum aria-label (REQ-009)', () => {
+		render(WorldBand, { entry: historical() });
+		const pill = document.querySelector('[data-testid="world-range"]');
+		expect(pill).not.toBeNull();
+		expect(pill?.textContent).toContain('1914–1918');
+		expect(pill?.getAttribute('aria-label')).toBe('Zeitraum: 1914 bis 1918');
+	});
+
+	it('degrades a RANGE with no end to the start year, no span pill, no crash (REQ-010)', () => {
+		render(WorldBand, { entry: historical({ eventDateEnd: undefined }) });
+		expect(document.querySelector('[data-testid="world-range"]')).toBeNull();
+		expect(document.body.textContent).toContain('Erster Weltkrieg');
+		expect(document.body.textContent).toContain('1914');
+	});
+});