feat(stammbaum): add clampZoom with resolved 0.25–3.0 zoom bounds (#692)

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

frontend/src/lib/person/genealogy/panZoom.test.ts

@@ -0,0 +1,26 @@
+import { describe, it, expect } from 'vitest';
+import { clampZoom, MIN_ZOOM, MAX_ZOOM } from './panZoom';
+
+describe('clampZoom', () => {
+	it('returns the value unchanged when within range', () => {
+		expect(clampZoom(1)).toBe(1);
+		expect(clampZoom(0.5)).toBe(0.5);
+		expect(clampZoom(2.75)).toBe(2.75);
+	});
+
+	it('clamps below MIN_ZOOM up to MIN_ZOOM', () => {
+		expect(clampZoom(0.1)).toBe(MIN_ZOOM);
+		expect(clampZoom(0)).toBe(MIN_ZOOM);
+		expect(clampZoom(-5)).toBe(MIN_ZOOM);
+	});
+
+	it('clamps above MAX_ZOOM down to MAX_ZOOM', () => {
+		expect(clampZoom(5)).toBe(MAX_ZOOM);
+		expect(clampZoom(3.0001)).toBe(MAX_ZOOM);
+	});
+
+	it('exposes the resolved zoom bounds', () => {
+		expect(MIN_ZOOM).toBe(0.25);
+		expect(MAX_ZOOM).toBe(3.0);
+	});
+});