Add CodeNotImplemented error constant (HTTP 501) for planned but unavailable features.
Refactor WebSocket packet handling from JSON to binary protocol for improved efficiency:
New packet structure:
- PacketTypeSuccess (0x00): [type:1][json_data:var]
- PacketTypeError (0x01): [type:1][code_len:1][code:var][msg_len:2][msg:var][details_len:2][details:var]
- PacketTypeData (0x02): Reserved for future use
Update SendErrorPacket:
- Build binary error packets with length-prefixed fields
- Use WriteMessage with websocket.BinaryMessage
Update SendSuccessPacket:
- Marshal data to JSON then wrap in binary packet
- Eliminates "success" wrapper field for cleaner protocol
Add helper functions:
- NewNotImplemented(feature) - Standard 501 error
- NewNotImplementedWithIssue(feature, issueURL) - 501 with GitHub reference
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