Function encode_header_into 

pub fn encode_header_into(out: &mut [u8], hdr: Header) -> Result<usize, Error>

Encode a v1 header into out.

Contract

• Writes exactly V1_HEADER_LEN bytes on success and returns V1_HEADER_LEN.
• Currently requires hdr.header_len == V1_HEADER_LEN.
• Rejects unknown flag bits for deterministic behavior.
• Validates hdr.header_len + hdr.payload_len <= SLOT_CAPACITY.
• Does not write any payload bytes; callers are expected to write the payload after the header.

Errors

Returns Error::BufferTooSmall if out is shorter than V1_HEADER_LEN. Returns Error::BadHeaderLen, Error::BadPayloadLen, or Error::UnknownFlags if the header is structurally invalid.