Tray Scan

Accept ONE tray photograph, segment individual stones, and stream per-stone identification results via SSE.

Response stream events:

• start : {"image_width", "image_height"}
• detected : {"stones": [{"index","x","y","width","height","confidence"}]}
• stone : {"index","result":{"species",..., "market_insights":...}}
• error : {"index","message"}
• complete : {"total_detected","total_identified","elapsed_ms"}