engine: headless pictures draw the analysis setup's symbols

view::scene draws the boundary conditions, loads and interactions as the
viewport does (muted over a results plot) and frames them; Shown.sampled and
the MCP picture info's sampled_symbols list definitions drawn with a sample.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

crates/mcp/src/backend.rs

@@ -119,6 +119,7 @@ impl Backend for Headless {
             "height": h,
             "results": shown.step.map(|step| json!({"step": step, "scale": shown.scale, "contour": contour})),
             "undrawn_bodies": shown.failed,
+            "sampled_symbols": shown.sampled,
         });
         Ok(Picture { png, info })
     }