buildCidRecordHeader

ubyte[] buildCidRecordHeader(ubyte  contentType,  ushort  epoch,  ulong  seqNum,
       DtlsConnectionId cid,  ushort  payloadLen)

Builds a DTLS 1.3 record header with optional CID.

Per RFC 9147 §4.2.2 there is no CID-length byte on the wire: the connection ID bytes are carried directly and the receiver slices its own negotiated CID length.

Parameters

`contentType`	Record content type.
`epoch`	Record epoch.
`seqNum`	48-bit sequence number.
`cid`	Connection ID to include (none = no CID field).
`payloadLen`	Payload length.

Returns

Serialized record header bytes.