Use unsigned atom sizes and counts

[seedds]

Summary

• parse MP4 atom sizes as unsigned 32/64-bit values
• parse AAVD sample lengths and MDAT count/length fields as unsigned values
• preserve signed reads for FourCC/tag fields only

Why

Large .aaxc files can contain mdat atoms larger than 2 GiB. Reading those sizes as signed integers turns values like 0x8f3354dd into negative numbers, which later triggers ValueError: read length must be non-negative or -1 during decryption.

Verification

• python3 -m py_compile src/snowcrypt/snowcrypt.py
• validated the patched parser reads a failing file's top-level mdat size as 2402505949 instead of a negative value
• validated the same patch in the installed package by successfully decrypting a previously failing 2.4 GB .aaxc file end-to-end

[snowskeleton]

Thanks for the PR! This also probably fixes #5