Fix HSC zeropoint decoding inverse

[tom-hehir]

Summary

• make HSC decoding multiply by the same 27.0 zeropoint flux scale used during encoding
• avoid passing a Python float to torch.log10 through reverse_zeropoint
• add focused HSC scale, forward/backward round-trip, and non-HSC identity regression tests

Forward preprocessing divides flux by convert_zeropoint(27.0). The exact inverse is multiplication by that same scale; reverse_zeropoint instead converts a scale into a zeropoint and is not the inverse flux operation.

Validation

• .venv/bin/pytest tests/codecs/test_image_codec.py -q (13 passed)
• pre-commit run on both changed files (all hooks passed)
• git diff --check

[tom-hehir]

LGTM