perf(shipwrecks): draw the wreck markers from one billboard collection, on the wreck, and rebuild them only on new data

- The ~8,000 markers were one Cesium entity each; they are now one
  BillboardCollection (as FIRMS, vessels and wildlife sightings do), ordered
  at the bottom of the shared sprite stack so moving traffic draws over it.
- eyeOffset.z was the death toll in metres, so a 1,500-death wreck's marker
  sat 1.5 km toward the camera and clipped or drifted when close. It is gone;
  markers are added by rising toll, so a bigger toll still draws on top.
- The page asks every 6 h but the proxy changes data at most daily: the proxy
  now sends a version that changes with either source's data, and the layer
  rebuilds only when it does.
- Adds a controller test: enable/disable, teardown, draw order, unchanged
  snapshots, a late response after turning off, and the period closing an
  open card.

server/providers/shipwrecks.js

@@ -143,7 +143,12 @@ export function shipwrecksProxy({
       };
     }
     return {
-      value: { fetchedAt, sources: sourceFlags, records: snapshot.records },
+      value: {
+        fetchedAt,
+        version: key,
+        sources: sourceFlags,
+        records: snapshot.records,
+      },
       stale,
     };
   }