Replace encode_message with encode_sub_message for protobuf submessage encoding.
For repeated submessage fields, encode_sub_message uses a backpatch approach:
writes field tag, reserves 1 byte for length varint, encodes the body, then
backpatches the actual length. For bodies >= 128 bytes, shifts the body forward
to make room for a multi-byte varint. This eliminates 2 of 3 calculate_size()
calls per repeated submessage element.
For singular submessage fields, encode_sub_message uses calculate_size() upfront
to skip empty submessages without writing to the buffer, preserving the debug
size check.
For the BLE advertisement proxy hot path (16 advertisements per batch), this
reduces calculate_size() calls from 48 to 16 per flush.
J. Nick Koston
a7100beb6f