Create and use WebRTC data channels with configurable reliability and ordering

Verified steps

1. After creating an RTCPeerConnection, call createDataChannel('label', options) on the offering peer before generating an offer; options can include ordered: false for unordered delivery and maxRetransmits: 0 for fully unreliable (useful for game state), or leave defaults for reliable ordered delivery.
2. The answering peer listens for the RTCPeerConnection 'datachannel' event to receive the remote channel; do not call createDataChannel on the answering side for the same label.
3. Wait for the channel's 'open' event before sending data; sending before the channel opens silently discards messages in some implementations.
4. Send binary data (ArrayBuffer or Blob) for efficiency; strings are valid but incur UTF-8 encoding overhead. Keep individual messages below the SCTP PMTU to avoid fragmentation — a common safe size is under 16 KB.
5. For large transfers, implement your own chunking: break the payload into chunks, send them sequentially, and reassemble on the receiver using a sequence number you include in each chunk.
6. To negotiate a data channel out-of-band (avoiding the SDP renegotiation for the channel itself), set negotiated: true and id: <same integer on both sides> in the options on both peers before connecting.