feat(street-level): KartaView as a third, keyless imagery provider

Coverage lines from KartaView's z13-14 JSON tiles (OSM ways with photos,
chained end to start), the photo nearest a clicked line, cones along its
sequence, and a viewer for flat photos and 360° panoramas (a small WebGL
equirectangular renderer, no new dependency) with ‹ › along the sequence.

Mapillary's camera-driven line coverage and sequence cones move to the
provider-neutral core (lineCoverage.js, sequenceCones.js) with the tile
fetch, zoom ladder, colours and pick prefixes injected; Mapillary keeps
thin wrappers and its tests pass unchanged.

KartaView quirks handled: api.kartaview.org (the documented
api.openstreetcam.org serves empty tiles), photo URLs that name storage1
for photos stored elsewhere, and a photo API that answers 429 to more than
about three requests at once (two in flight, retried with backoff).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/testSupport/kartaViewFakes.mjs

@@ -0,0 +1,60 @@
+// DOM stand-ins for the KartaView viewer's tests (no browser in Node).
+
+/** A DOM element with what the viewer touches: children, events, attributes. */
+export function fakeElement(tag) {
+  const listeners = new Map();
+  const element = {
+    tagName: tag.toUpperCase(),
+    children: [],
+    parentNode: null,
+    style: {},
+    dataset: {},
+    attributes: {},
+    hidden: false,
+    disabled: false,
+    clientWidth: 400,
+    clientHeight: 250,
+    classList: {
+      values: new Set(),
+      add: (name) => element.classList.values.add(name),
+      remove: (name) => element.classList.values.delete(name),
+    },
+    append(...nodes) {
+      for (const node of nodes) {
+        node.parentNode = element;
+        element.children.push(node);
+      }
+    },
+    remove() {
+      const list = element.parentNode?.children;
+      if (list) list.splice(list.indexOf(element), 1);
+      element.parentNode = null;
+    },
+    setAttribute: (name, value) => (element.attributes[name] = value),
+    removeAttribute: (name) => {
+      delete element.attributes[name];
+      if (name === 'src') element.src = '';
+    },
+    addEventListener: (type, handler) => listeners.set(type, handler),
+    removeEventListener: (type) => listeners.delete(type),
+    setPointerCapture() {},
+    /** Fire an event at the element's listener. */
+    fire(type, event = {}) {
+      return listeners.get(type)?.({
+        preventDefault() {
+          this.defaultPrevented = true;
+        },
+        stopPropagation() {},
+        ...event,
+      });
+    },
+  };
+  return element;
+}
+
+export function fakeHost() {
+  const doc = { createElement: (tag) => fakeElement(tag) };
+  const host = fakeElement('div');
+  host.ownerDocument = doc;
+  return host;
+}