naturalEarthRegions.js and neighborhoodPolygons.js each carried an
isNode branch that dynamically imported node:fs to read their bundled JSON
packs, because a plain dynamic JSON import needs an import attribute in
Node. Vite only warns about externalizing node:fs for the browser, so both
warnings survived every production build and the runtime boundary rested on
an environment guard.
Give both loaders the import attribute instead. Vite bundles the JSON as a
module exactly as before (the emitted regions/marine/san-francisco chunks
are byte-identical), Node loads the same files under node:test, and the
isNode split disappears.
Add a source-boundary test so a node: import cannot silently return to a
browser-built module.
Closes#34
Ethan Stoner