feat: add useLensFrameMetrics hook

[msilivonik-sc]

Summary

• Add useLensFrameMetrics hook that exposes lens rendering performance data (ComputedFrameMetrics) from the CameraKit SDK's session.metrics API
• The hook manages the full LensPerformanceMeasurement lifecycle: begins measurement when a session is available, polls at a configurable interval, resets on lens change, and calls end() on cleanup to prevent memory leaks
• Supports an enabled option (defaults to true) so consumers can toggle measurement without breaking rules of hooks

Usage

import { useLensFrameMetrics } from "@snap/react-camera-kit";

function PerformanceOverlay() {
  const metrics = useLensFrameMetrics({ interval: 500 });

  if (!metrics) return null;

  return (
    <div>
      <p>FPS: {metrics.avgFps.toFixed(1)}</p>
      <p>Frame time: {metrics.lensFrameProcessingTimeMsAvg.toFixed(1)}ms</p>
    </div>
  );
}

Test plan

• 11 new unit tests covering: no session, begin/end lifecycle, polling, lens change reset, unmount cleanup, session re-bootstrap, enabled/disabled toggling, session loss
• Full test suite passes (301 tests)
• Build passes (ESM + CJS)
• Verify in demo app: hook returns metrics when lens is active, returns undefined when no session