fix: measure traffic inside the requested circle and report missing tiles

Traffic figures for a radius area counted roads anywhere in its bounding
box, and a tile that failed to load silently changed them. Roads are now cut
to the circle, and the flow source reports per request whether any tile
failed so the answer can say its figures are partial.

src/tools/queries/mobility.test.mjs

@@ -428,3 +428,56 @@ test('bike share names the systems it did not search', async () => {
     /more not searched: .*; use a smaller area\)\.$/,
   );
 });
+
+test('traffic counts only roads inside a radius area and reports missing tiles', async () => {
+  const flow = (segments, partial = false) => ({
+    getStatus: async () => ({ hasKey: true }),
+    fetchFlowDetail: async () => ({ segments, partial }),
+  });
+  const call = (traffic, area) =>
+    composeCatalog({ tools: coreTools, services: { traffic } }).call(
+      'get_traffic_flow',
+      { area },
+    );
+  const circle = { lat: 0, lon: 0, radius_km: 10 };
+  // In the circle's bounding box, but about 12.6 km from its center.
+  const corner = [
+    {
+      coords: [
+        [0.08, 0.08],
+        [0.085, 0.085],
+      ],
+      closure: true,
+      trafficLevel: 1,
+    },
+  ];
+  const outside = await call(flow(corner), circle);
+  assert.equal(outside.data.closed_km, 0);
+  assert.equal(outside.data.measured_km, 0);
+  // A 20 km road through the center keeps the 20 km inside the circle.
+  const across = [
+    {
+      coords: [
+        [-0.2, 0],
+        [0.2, 0],
+      ],
+      trafficLevel: 1,
+    },
+  ];
+  const crossing = await call(flow(across), circle);
+  assert.ok(Math.abs(crossing.data.measured_km - 20) < 0.2);
+  // A road cutting the circle with both ends outside it.
+  const chord = [
+    {
+      coords: [
+        [-0.2, 0.05],
+        [0.2, 0.05],
+      ],
+      trafficLevel: 1,
+    },
+  ];
+  assert.ok((await call(flow(chord), circle)).data.measured_km > 10);
+  const partial = await call(flow(across, true), circle);
+  assert.equal(partial.data.partial, true);
+  assert.match(partial.summary, /Some map tiles did not load/);
+});