Self-built wire-frame codec for SPG.
Frame layout (little-endian):
+-----------------+--------+----------------------------+
| payload_len:u32 | op:u8 | payload[payload_len bytes] |
+-----------------+--------+----------------------------+
Header is always [FRAME_HEADER_LEN] bytes. Maximum payload is
[MAX_PAYLOAD] bytes; oversized frames are rejected before allocation.
Endianness is little-endian everywhere (modern CPUs are LE; the protocol is self-defined so we drop the PG/MySQL big-endian baggage).