Initiate a flight search and stream normalised solutions back as Server-Sent Events. Each data: event is a JSON-encoded flight solution object whose id is a canonical Mercury UUID (see POST /v1/flights/fares). The stream ends with event: done.

Send presentation: "grouped" on a two-leg (round-trip) itinerary to receive event: outbound-group frames instead — one per physical outbound, each carrying a from-price and its priced return options. See docs/adr/MER-1488-flight-grouped-search.md.

Frame vocabulary (event: names on this stream):

• solution — one normalised flight solution (default framing).
• outbound-group — one physical outbound with its priced return options (grouped framing only).
• done — terminal frame; the stream is exhausted (data: {}).
• error — terminal frame on failure; data: {"code": ..., "message": ...}.