Add BRouter mock for deterministic E2E tests

- brouter-mock.ts: Canned route responses for 2 and 3 waypoint sessions,
  plus latLngToPixel helper for pixel-precise map interactions
- planner.test.ts: Route split, map zoom, and overnight tests now use
  mockBRouter() for instant deterministic responses instead of real
  BRouter — fixes all 3 flaky tests
- Used .first() for sidebar km locator to avoid strict mode violations
  when both header summary and stats footer match

All 38 E2E tests pass in 9.7s (was 34s+ with real BRouter).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

e2e/fixtures/brouter-mock.ts

@@ -0,0 +1,97 @@
+import type { Page } from "@playwright/test";
+
+/**
+ * Canned enriched route for 3 nearby Berlin waypoints:
+ *   [52.520, 13.405] → [52.516, 13.377] → [52.510, 13.390]
+ *
+ * ~3.2km total, 6 track points with elevation.
+ */
+const MOCK_ROUTE_3WP = {
+  geojson: {
+    type: "FeatureCollection",
+    features: [{
+      type: "Feature",
+      geometry: {
+        type: "LineString",
+        coordinates: [
+          [13.405, 52.520, 34], [13.398, 52.519, 36], [13.391, 52.518, 38],
+          [13.384, 52.517, 40], [13.377, 52.516, 42],
+          [13.379, 52.514, 40], [13.382, 52.513, 38],
+          [13.385, 52.512, 36], [13.388, 52.511, 35], [13.390, 52.510, 34],
+        ],
+      },
+      properties: {},
+    }],
+  },
+  coordinates: [
+    [13.405, 52.520, 34], [13.398, 52.519, 36], [13.391, 52.518, 38],
+    [13.384, 52.517, 40], [13.377, 52.516, 42],
+    [13.379, 52.514, 40], [13.382, 52.513, 38],
+    [13.385, 52.512, 36], [13.388, 52.511, 35], [13.390, 52.510, 34],
+  ],
+  segmentBoundaries: [0, 4],
+  totalLength: 3200,
+  totalAscend: 8,
+  surfaces: [],
+};
+
+/**
+ * Canned enriched route for 2 nearby Berlin waypoints:
+ *   [52.520, 13.405] → [52.515, 13.351]
+ *
+ * ~3.8km total, 6 track points with elevation.
+ */
+const MOCK_ROUTE_2WP = {
+  geojson: {
+    type: "FeatureCollection",
+    features: [{
+      type: "Feature",
+      geometry: {
+        type: "LineString",
+        coordinates: [
+          [13.405, 52.520, 34], [13.394, 52.519, 36], [13.383, 52.518, 40],
+          [13.372, 52.517, 38], [13.361, 52.516, 36], [13.351, 52.515, 35],
+        ],
+      },
+      properties: {},
+    }],
+  },
+  coordinates: [
+    [13.405, 52.520, 34], [13.394, 52.519, 36], [13.383, 52.518, 40],
+    [13.372, 52.517, 38], [13.361, 52.516, 36], [13.351, 52.515, 35],
+  ],
+  segmentBoundaries: [0],
+  totalLength: 3800,
+  totalAscend: 6,
+  surfaces: [],
+};
+
+/**
+ * Mock the BRouter route API to return instant, deterministic responses.
+ * Selects the appropriate canned response based on waypoint count.
+ */
+export async function mockBRouter(page: Page) {
+  await page.route("**/api/route", async (route) => {
+    const body = route.request().postDataJSON();
+    const wpCount = body?.waypoints?.length ?? 0;
+    const mock = wpCount >= 3 ? MOCK_ROUTE_3WP : MOCK_ROUTE_2WP;
+    await route.fulfill({
+      status: 200,
+      contentType: "application/json",
+      body: JSON.stringify(mock),
+    });
+  });
+}
+
+/**
+ * Convert a lat/lon to pixel coordinates on the Leaflet map.
+ * Requires MapExposer to have set window.__leafletMap.
+ */
+export async function latLngToPixel(page: Page, lat: number, lon: number): Promise<{ x: number; y: number }> {
+  return page.evaluate(([la, ln]) => {
+    const map = (window as any).__leafletMap;
+    if (!map) throw new Error("__leafletMap not available");
+    const point = map.latLngToContainerPoint([la, ln]);
+    return { x: point.x, y: point.y };
+  }, [lat, lon]);
+}