Function decode_header 

pub fn decode_header(input: &[u8]) -> Result<(Header, &[u8]), Error>

Decode a v1 header from input.

Returns the parsed header and a slice of the payload bytes.

Contract

• Accepts header_len >= V1_HEADER_LEN and ignores appended header bytes it does not understand.
• Rejects unknown flag bits for deterministic behavior.
• Validates header_len + payload_len <= SLOT_CAPACITY.
• Returns a payload slice that borrows from input.

Errors

Returns Error::Truncated if input does not contain enough bytes for the declared header or payload. Returns Error::BadMagic, Error::BadHeaderLen, Error::BadPayloadLen, or Error::UnknownFlags if the envelope is structurally invalid.