render_map_tool: tell the model what the user did on the map #287

Problem

What the user does on the map never reaches the model: toggling traffic or satellite (#278), picking a route, clicking a place, or collapsing the panel. So a follow-up like "how long is that one?" or "is the traffic bad there?" has nothing to go on.

Proposal

When the user acts on the map, send a short state update to the host as model context (MCP Apps ui/update-model-context, or ChatGPT's widget state). For example: "User selected route 2 (24 min, 9.1 mi)", "Traffic layer on", "User clicked Blue Bottle Coffee (mapbox_id …)". Keep it to the latest state, not a log.

Notes

• Check what each host supports and how it reaches the model.
• Make it compact: a sentence or a small object, never geometry.
• Supports click-to-ask (render_map_tool: click a place to ask the model about it #284) and the route picker (render_map_tool: show and pick route alternatives on the map #285).