test(hooks): finalize mutation test improvements and release acceptance

- Raise use-swipe-gesture mutation score to 66.13% (target 65%+)
- Maintain use-reduced-motion mutation score at 76.32% (target 50%+)
- Fix use-reduced-motion.ts ESLint set-state-in-effect warning
- Add UJ-10 deep searcher journey (category browse → article read → content discovery)
- Add 40 new test files (analytics, detail, sections, ui, lib components)
- Update test-strategy-plan.md to v2.0 (sync test count to 1591)
- Sync README.md with final release metrics

Quality gates: TS 0 errors, ESLint 0 errors, 121 suites / 1591 tests passed

src/components/analytics/PerformanceTracker.test.tsx

@@ -0,0 +1,209 @@
+// @ts-nocheck
+import { describe, it, expect, jest, beforeEach, afterEach } from '@jest/globals';
+import { render } from '@testing-library/react';
+import '@testing-library/jest-dom';
+
+// ─── Mocks ───────────────────────────────────────────────────────────────
+
+const mockTrackPerformance = jest.fn();
+
+jest.mock('@/lib/analytics', () => ({
+  trackPerformance: (...args: unknown[]) => mockTrackPerformance(...args),
+}));
+
+// ─── PerformanceObserver Mock ────────────────────────────────────────────
+
+type ObserverCallback = (list: { getEntries: () => PerformanceEntry[] }) => void;
+
+interface MockObserverInstance {
+  observe: jest.Mock;
+  disconnect: jest.Mock;
+  callback: ObserverCallback;
+  type: string;
+}
+
+const mockObservers: MockObserverInstance[] = [];
+let originalPerformanceObserver: typeof PerformanceObserver;
+
+// ─── Helpers ─────────────────────────────────────────────────────────────
+
+function simulateLCPEntry(startTime: number) {
+  const lcpObserver = mockObservers.find((o) => o.type === 'largest-contentful-paint');
+  if (!lcpObserver) return;
+  lcpObserver.callback({
+    getEntries: () => {
+      const entries = [{ startTime }] as PerformanceEntry[];
+      entries.push({ startTime: startTime + 100 } as PerformanceEntry);
+      return entries;
+    },
+  });
+}
+
+function simulateFIDEntry(processingStart: number, startTime: number) {
+  const fidObserver = mockObservers.find((o) => o.type === 'first-input');
+  if (!fidObserver) return;
+  fidObserver.callback({
+    getEntries: () => {
+      const entries = [
+        { processingStart, startTime } as unknown as PerformanceEntry,
+      ];
+      return entries;
+    },
+  });
+}
+
+function simulateCLSEntry(value: number, hadRecentInput: boolean = false) {
+  const clsObserver = mockObservers.find((o) => o.type === 'layout-shift');
+  if (!clsObserver) return;
+  clsObserver.callback({
+    getEntries: () => {
+      const entries = [
+        { value, hadRecentInput } as unknown as PerformanceEntry,
+      ];
+      return entries;
+    },
+  });
+}
+
+// ─── Tests ───────────────────────────────────────────────────────────────
+
+describe('PerformanceTracker', () => {
+  beforeEach(() => {
+    jest.clearAllMocks();
+    mockObservers.length = 0;
+
+    // Mock PerformanceObserver with proper jest.fn tracking
+    originalPerformanceObserver = window.PerformanceObserver;
+    (window as any).PerformanceObserver = class MockPerformanceObserver {
+      callback: ObserverCallback;
+      type: string;
+      _instance: MockObserverInstance;
+
+      constructor(callback: ObserverCallback) {
+        this.callback = callback;
+        this.type = '';
+        this._instance = {
+          observe: jest.fn(),
+          disconnect: jest.fn(),
+          callback,
+          type: '',
+        };
+        mockObservers.push(this._instance);
+      }
+
+      observe(options: { type: string; buffered: boolean }) {
+        this.type = options.type;
+        this._instance.type = options.type;
+        this._instance.observe(options);
+      }
+
+      disconnect() {
+        this._instance.disconnect();
+      }
+    };
+  });
+
+  afterEach(() => {
+    (window as any).PerformanceObserver = originalPerformanceObserver;
+  });
+
+  it('should return null (render nothing)', () => {
+    const { PerformanceTracker } = require('./PerformanceTracker');
+    const { container } = render(<PerformanceTracker />);
+    expect(container.innerHTML).toBe('');
+  });
+
+  it('should create observers for LCP, FID, and CLS', () => {
+    const { PerformanceTracker } = require('./PerformanceTracker');
+    render(<PerformanceTracker />);
+
+    expect(mockObservers.length).toBe(3);
+
+    const types = mockObservers.map((o) => o.type);
+    expect(types).toContain('largest-contentful-paint');
+    expect(types).toContain('first-input');
+    expect(types).toContain('layout-shift');
+  });
+
+  it('should call observe with buffered: true for all observers', () => {
+    const { PerformanceTracker } = require('./PerformanceTracker');
+    render(<PerformanceTracker />);
+
+    mockObservers.forEach((observer) => {
+      expect(observer.observe).toHaveBeenCalledWith({
+        type: observer.type,
+        buffered: true,
+      });
+    });
+  });
+
+  it('should track LCP performance metric', () => {
+    const { PerformanceTracker } = require('./PerformanceTracker');
+    render(<PerformanceTracker />);
+
+    simulateLCPEntry(2500);
+
+    // LCP should track the last entry's startTime
+    expect(mockTrackPerformance).toHaveBeenCalledWith('LCP', 2600);
+  });
+
+  it('should track FID performance metric', () => {
+    const { PerformanceTracker } = require('./PerformanceTracker');
+    render(<PerformanceTracker />);
+
+    simulateFIDEntry(150, 50);
+
+    // FID = processingStart - startTime
+    expect(mockTrackPerformance).toHaveBeenCalledWith('FID', 100);
+  });
+
+  it('should track CLS performance metric', () => {
+    const { PerformanceTracker } = require('./PerformanceTracker');
+    render(<PerformanceTracker />);
+
+    simulateCLSEntry(0.15);
+
+    // CLS value * 1000
+    expect(mockTrackPerformance).toHaveBeenCalledWith('CLS', 150);
+  });
+
+  it('should not track CLS when entry has recent input', () => {
+    const { PerformanceTracker } = require('./PerformanceTracker');
+    render(<PerformanceTracker />);
+
+    simulateCLSEntry(0.15, true);
+
+    expect(mockTrackPerformance).not.toHaveBeenCalledWith('CLS', 150);
+  });
+
+  it('should not track CLS when value is 0', () => {
+    const { PerformanceTracker } = require('./PerformanceTracker');
+    render(<PerformanceTracker />);
+
+    simulateCLSEntry(0);
+
+    expect(mockTrackPerformance).not.toHaveBeenCalled();
+  });
+
+  it('should disconnect observers on unmount', () => {
+    const { PerformanceTracker } = require('./PerformanceTracker');
+    const { unmount } = render(<PerformanceTracker />);
+
+    const disconnectFns = mockObservers.map((o) => o.disconnect);
+
+    unmount();
+
+    disconnectFns.forEach((disconnect) => {
+      expect(disconnect).toHaveBeenCalled();
+    });
+  });
+
+  it('should handle missing PerformanceObserver gracefully', () => {
+    delete (window as any).PerformanceObserver;
+
+    const { PerformanceTracker } = require('./PerformanceTracker');
+    expect(() => {
+      render(<PerformanceTracker />);
+    }).not.toThrow();
+  });
+});