ConvNeXt V2 native classifier

Adapted from facebookresearch/ConvNeXt-V2
https://github.com/facebookresearch/ConvNeXt-V2
Commit: 2553895753323c6fe0b2bf390683f5ea358a42b9
Files: models/convnextv2.py; dense LayerNorm and GRN from models/utils.py.
Code license: MIT. Official ImageNet pretrained/fine-tuned weights:
CC-BY-NC-4.0, redistributed separately on Hugging Face with the full license.

Modifications: removed sparse pretraining and timm runtime dependencies;
added native family registration, classification head reset, conversion,
preprocessing reuse, and supervised fine-tuning integration. Dense forward
arithmetic and official tensor names are preserved. The supervised recipe
uses zero stochastic depth; no FCMAE pretraining implementation is included.

MIT License
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