Fix AdaptiveDetector min_scene_len target frame check

[aouxwoux]

Summary

• apply AdaptiveDetector's min_scene_len check to the buffered target frame instead of the delayed/current frame
• add a synthetic regression test for two adaptive spikes that are only four target frames apart

Fixes #408

Verification

• python -m pytest tests/test_detectors.py::test_adaptive_detector_uses_target_frame_for_min_scene_len tests/test_detectors.py -q
• python -m ruff check scenedetect/detectors/adaptive_detector.py tests/test_detectors.py
• git diff --check