encodeFixed64Field function
Encodes a fixed64 field (wire type 1) into buffer.
Proto3 rule: if value is 0, nothing is written.
The unsigned 64-bit value is serialized as a field tag followed by 8
little-endian bytes.
Returns the same buffer list, possibly with bytes appended.
Implementation
List<int> encodeFixed64Field(List<int> buffer, int fieldNumber, int value) {
if (value == 0) return buffer;
encodeTag(buffer, fieldNumber, 1);
encodeFixed64(buffer, value);
return buffer;
}