features2d: add tests and benchmarks for features2d

[kalwalt]

Add comprehensive unit tests and benchmarks for both FAST and ORB.

• For FAST:
  • test detection on synthetic/image patterns
  • test NMS correctness
• For ORB:
  • test pyramid construction, angle computation, and descriptor length
  • property-based/edge-case tests
    Add benches/features2d_bench.rs using Criterion.

fn bench_fast(c: &mut Criterion) {
    // ...
}
fn bench_orb_detect_and_compute(c: &mut Criterion) {
    // ...
}

Compare results to OpenCV where practical (but do not require exact parity).

References: https://docs.opencv.org/4.10.0/d5/d51/group__features2d__main.html