Load the layer displays lazily and rebase the main-chunk budget

Every layer display was imported statically from the manifest, so all of
them sat in the Cesium-mode main chunk. They are re-exported from one
module, src/app/layers/displays.js, which the manifest loads with one
import() before the catalog is built, on every engine; rendererFor stays
synchronous after that. One chunk beats per-family chunks here: every
start loads every display, and split chunks compress worse. CCTV's world
click gesture moves out of its display, which the app imported only for
it. Tests that build layers without the application await the loads.

Keyless, gzip-9: the main chunk falls from 2,113,641 to 2,066,560
bytes, below the 2,090,274 of the merge with main this branch carries.
A cold Cesium start loads about 7 KB more JS in total over four requests,
with the same time to a ready page. The budget's baseline moves to the
new size; it had never absorbed main's own growth.

scripts/main-chunk-baseline.json

@@ -1,6 +1,6 @@
 {
-  "gzipBytes": 2021566,
-  "allowanceBytes": 20216,
-  "measuredAt": "merge of origin/main ac28a11e, keyless build (GOOGLE_MAPS_API_KEY and CESIUM_ION_TOKEN empty), entry index-ChmLBAMR.js, 6 220 779 bytes, gzip level 9; main itself grew 93 873 bytes gzip-9 from the merge base (1 853 283 to 1 947 156)",
+  "gzipBytes": 2066560,
+  "allowanceBytes": 20666,
+  "measuredAt": "HEAD 47d63128 with the layer displays loaded through import() (src/app/layers/displays.js), keyless build (GOOGLE_MAPS_API_KEY and CESIUM_ION_TOKEN empty), entry index-DEjtUM5w.js, 6 322 899 bytes, gzip level 9; HEAD 47d63128 itself measured 2 113 641 and the merge of main 92c8f8d9 2 090 274",
   "allowance": "1% of the baseline; growth past it fails npm run check:main-chunk until the baseline is raised on purpose"
 }