Function decode_header 

pub fn decode_header(bytes: &[u8]) -> Result<EnvelopeHeader, DecodeError>

Decode an envelope header from the front of bytes, following the frozen-prefix discipline:

1. need at least the 5-byte prefix to read len + ver;
2. dispatch the full header length on ver;
3. need the full header present; then parse the rest.

Never panics on malformed input — returns a typed DecodeError.