Update default checkpoint to v4

Update default model to v4 (docs_v2 + general_textline_data, 53k images; merge/expand/drop-non-text postprocessing)

Document the v2 default, pinned v1/v2 files, and threshold presets

Promote v2 (docs_v2 fine-tune, best epoch 2, val F1 0.9328) to the default weights

Promote v2 (docs_v2 fine-tune, best epoch 2, val F1 0.9328) to the default weights

Promote v2 (docs_v2 fine-tune, best epoch 2, val F1 0.9328) to the default weights

Promote v2 (docs_v2 fine-tune, best epoch 2, val F1 0.9328) to the default weights

Preserve v1 weights as khmer_dbnet_mobilenetv3_v1.pth before default is updated to v2

Preserve v1 weights as khmer_dbnet_mobilenetv3_v1.onnx before default is updated to v2

Upload khmer_dbnet_mobilenetv3.pth with huggingface_hub

Upload khmer_dbnet_mobilenetv3.onnx with huggingface_hub

Upload README.md with huggingface_hub

initial commit