Fix TypeError in save_meridian for .textproto & .txtpb formats

[farok-amo]

Summary

Fixes TypeError when saving Meridian models in textproto format

Problem

When calling save_meridian() with a .textproto or .txtpb file path,
the function raises:
TypeError: a bytes-like object is required, not 'str'

This occurs because:

• The file is opened in binary mode ('wb') on line 355
• text_format.MessageToString() returns a string, not bytes
• Writing a string to a binary file causes the TypeError

Solution

Encode the string to UTF-8 bytes before writing:

f.write(text_format.MessageToString(serialized_kernel).encode('utf-8'))

Testing

• Tested manually with .textproto format
• Confirms the TypeError is resolved
• .binpb format unchanged (already returns bytes)