feat(street-level): provider contract with Mapillary as the first provider

Street Level is now a provider-neutral layer modelled on the iD editor's
photo overlay. The core owns what every imagery provider shares: the
enable state and a switch per provider, the imagery filter (360°/flat and
captured-since in relative days), one click handler routed by pick prefix,
one viewer host that mounts whichever provider's viewer adapter owns the
open image, the position marker, camera follow, one on-globe credit per
active provider, and share-link parameters (one bit per provider plus the
filter). Providers implement the contract in registry.js: coverage,
sequences, nearest-image lookup and a viewer adapter emitting a common
pose. Mapillary is the only registered provider; Google Street View,
KartaView and Panoramax slot in beside it.

Panel: a PROVIDERS chip row (one chip per registered provider, keyless
chips read as errors), relative-day SINCE options, a provider-neutral
caption whose link label follows the provider, and no vendor mark in the
header. The panel inner is a rail scroller like CCTV's.

Tests cover the registry, params codec, filter, pick router, UI-state
composition, the provider definition and coverage-tile decoding; the QA
harness switches the provider off and on and checks the filter.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

src/layers/streetLevel/view.js

@@ -0,0 +1,84 @@
+import * as Cesium from 'cesium';
+
+/**
+ * Camera height above the surface under the camera, in metres. Falls back to
+ * the ellipsoidal height when the globe has no height sample yet.
+ */
+export function cameraHeightAboveGround(viewer) {
+  const carto = viewer?.camera?.positionCartographic;
+  if (!carto) return null;
+  const ground = viewer.scene?.globe?.getHeight?.(carto);
+  return carto.height - (Number.isFinite(ground) ? ground : 0);
+}
+
+/**
+ * Visible bbox as [west, south, east, north] degrees, or null when the camera
+ * does not see the ground. A grid of screen rays is cast onto the ellipsoid
+ * and only the rays that hit count, so a view that includes the horizon (or a
+ * canvas whose frustum is stale) cannot inflate the box to the whole world;
+ * `computeViewRectangle` is the fallback when too few rays land.
+ */
+export function visibleBbox(viewer, { grid = 5 } = {}) {
+  const scene = viewer?.scene;
+  const camera = viewer?.camera;
+  if (!scene || !camera) return null;
+  const width = scene.canvas?.clientWidth || scene.canvas?.width || 0;
+  const height = scene.canvas?.clientHeight || scene.canvas?.height || 0;
+  const hits = [];
+  if (width > 0 && height > 0) {
+    const ellipsoid = scene.globe?.ellipsoid || Cesium.Ellipsoid.WGS84;
+    const point = new Cesium.Cartesian2();
+    for (let i = 0; i <= grid; i++) {
+      for (let j = 0; j <= grid; j++) {
+        point.x = (width * i) / grid;
+        point.y = (height * j) / grid;
+        let cartesian = null;
+        try {
+          cartesian = camera.pickEllipsoid(point, ellipsoid);
+        } catch {
+          cartesian = null;
+        }
+        if (!cartesian) continue;
+        const carto = Cesium.Cartographic.fromCartesian(cartesian, ellipsoid);
+        if (carto) hits.push(carto);
+      }
+    }
+  }
+  if (hits.length >= 4) {
+    let west = Infinity;
+    let south = Infinity;
+    let east = -Infinity;
+    let north = -Infinity;
+    for (const carto of hits) {
+      const lon = Cesium.Math.toDegrees(carto.longitude);
+      const lat = Cesium.Math.toDegrees(carto.latitude);
+      west = Math.min(west, lon);
+      east = Math.max(east, lon);
+      south = Math.min(south, lat);
+      north = Math.max(north, lat);
+    }
+    if (east - west > 0 && north - south > 0) return [west, south, east, north];
+  }
+  const rectangle = camera.computeViewRectangle?.(scene.globe?.ellipsoid);
+  if (!rectangle) return null;
+  return [
+    Cesium.Math.toDegrees(rectangle.west),
+    Cesium.Math.toDegrees(rectangle.south),
+    Cesium.Math.toDegrees(rectangle.east),
+    Cesium.Math.toDegrees(rectangle.north),
+  ];
+}
+
+/** Centre of the visible ground, or the camera's own footprint. */
+export function viewCentre(viewer) {
+  if (!viewer) return null;
+  const bbox = visibleBbox(viewer);
+  if (bbox)
+    return { lat: (bbox[1] + bbox[3]) / 2, lon: (bbox[0] + bbox[2]) / 2 };
+  const carto = viewer.camera?.positionCartographic;
+  if (!carto) return null;
+  return {
+    lat: Cesium.Math.toDegrees(carto.latitude),
+    lon: Cesium.Math.toDegrees(carto.longitude),
+  };
+}