Each leaf's dev page must hand its element assetsBase "/assets"; a boundary file and a table under
that base must answer 200 with bytes that JSON.parse (never an HTML fallback), and a bogus file
under it must be a real 404.
Reported from a laptop running `bun run dev`: under the Plugin Studio, maps drew with no boundary
polygons and UPG-100 had no posters. The built tree was fine, and every gate measured the built tree.
Cause: a dev leaf page is /__leaf/<bundle>/<profile>. A plugin that falls back to the page-relative
'./assets' asked for /__leaf/<bundle>/assets/…, and the leaf route answered with its own HTML at
HTTP 200, so JSON.parse failed and the map drew without our data. /<tree>/assets/ was not served
in dev at all. The built leaf never hits this because its page passes assetsBase "../../assets".
- dev-plugin serves the pool at /__leaf/…/assets/ and /<tree>/assets/ too, and the dev leaf page
passes assetsBase "/assets" in its profile-config, as the built leaf passes its own.
- resolveAssetsBase's last rung is '/assets' under the dev server (import.meta.env.DEV, false in
every build); the built tree keeps './assets'.
- check:dev-assets (headless): starts a short-lived dev server for this checkout, requires
boundaries/previews/fonts/images/tables to answer 200 and not HTML at every prefix a dev page can
produce, and requires the dev leaf to carry the root-absolute assetsBase. It fails on the parent
commit (4 prefixes and both leaves) and passes here.
