Add BDD100K detection dataset loader

[JaisalJain]

Closes #515

Adds a BDD100K detection dataset loader, following the same pattern as the existing YOLO loader.

Files changed

• perceptionmetrics/datasets/bdd100k.py: BDD100KDetectionDataset class and build_bdd100k_dataset() function
• perceptionmetrics/datasets/__init__.py: registers BDD100K in the REGISTRY
• tests/test_bdd100k.py: 5 mocked tests, no dataset download needed to run them
• examples/bdd100k_detection.py: shows how to load the dataset

Expected directory layout

<images_dir>/train/*.jpg
<images_dir>/val/*.jpg
<labels_dir>/train/*.json  (one JSON per image)
<labels_dir>/val/*.json

Tested on real data

Loaded 80,000 samples (70k train, 10k val). Ground truth boxes shown below.