test(timeline): guard the layer filter against navigation and fetch

A static boundary gate (mirroring the project's no-`{@html}` greps) that reads
TimelineFilters.svelte and /zeitstrahl/+page.svelte and fails if either ever
reintroduces goto(, url.searchParams, api.GET, or fetch( — the filter must stay
presentation-only and fully client-side.

Refs #780
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -0,0 +1,37 @@
+import { describe, it, expect } from 'vitest';
+import { readFileSync } from 'node:fs';
+import { fileURLToPath } from 'node:url';
+
+// REQ-001/002: the layer filter is presentation-only and fully client-side. It
+// must never navigate or fetch — the route derives the filtered view from
+// already-loaded data. This static guard mirrors the project's existing
+// grep-gates (e.g. the no-`{@html}` checks) and fails the build if a future
+// edit reintroduces navigation or a network call into either file.
+const read = (relative: string) =>
+	readFileSync(fileURLToPath(new URL(relative, import.meta.url)), 'utf8');
+
+const FILES = {
+	'TimelineFilters.svelte': read('./TimelineFilters.svelte'),
+	'/zeitstrahl/+page.svelte': read('../../routes/zeitstrahl/+page.svelte')
+};
+
+const FORBIDDEN: { label: string; pattern: RegExp }[] = [
+	{ label: 'goto(', pattern: /\bgoto\s*\(/ },
+	{ label: 'url.searchParams', pattern: /url\.searchParams/ },
+	{ label: 'api.GET', pattern: /\bapi\.GET\b/ },
+	{ label: 'fetch(', pattern: /\bfetch\s*\(/ }
+];
+
+describe('layer-filter boundary (REQ-001/002)', () => {
+	for (const [name, source] of Object.entries(FILES)) {
+		it(`${name} was found and is non-empty`, () => {
+			expect(source.length).toBeGreaterThan(0);
+		});
+
+		for (const { label, pattern } of FORBIDDEN) {
+			it(`${name} contains no ${label}`, () => {
+				expect(pattern.test(source), `${name} must not use ${label}`).toBe(false);
+			});
+		}
+	}
+});