Tag15693.set_uid() now:
- Accepts None (or no args) to generate a random valid UID with the
correct IC-specific prefix (E0 + mfg + IC type)
- Validates length (must be 8 bytes)
- Warns on prefix mismatch (wrong IC type for product-specific tags)
- Warns on non-NXP manufacturer code
- Warns on missing E0 prefix