fix(street-level): KartaView round-2 follow-ups — context loss, size cap, slow lookups

From the second IC8 pass (launch ready; P3s closed here):
- A lost WebGL context is redrawn on a new canvas (the old one only gives
  back its lost context), through the core, and only if no newer photo is
  opening; verified live with WEBGL_lose_context, twice.
- The 16 MB answer cap counts bytes while reading: KartaView sends
  chunked answers with no Content-Length, so the header check never fired.
- A lookup gets 8 s, then 15 s once more: KartaView sometimes sits on a
  lookup for 11 s and answers the same one in 3 s.

Left as follow-ups: cancelling abandoned sequence pages; finding a page
by binary search on the index when thousands of photos are missing.

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

src/testSupport/kartaViewFakes.mjs

@@ -30,6 +30,14 @@ export function fakeElement(tag) {
       if (list) list.splice(list.indexOf(element), 1);
       element.parentNode = null;
     },
+    replaceWith(next) {
+      const parent = element.parentNode;
+      if (!parent) return;
+      parent.children[parent.children.indexOf(element)] = next;
+      next.parentNode = parent;
+      next.ownerDocument = element.ownerDocument;
+      element.parentNode = null;
+    },
     setAttribute: (name, value) => (element.attributes[name] = value),
     removeAttribute: (name) => {
       delete element.attributes[name];