[frontend-contrib S2] P2 proof: literal lazy route loaders + normalizeFreshRouteModule

[rickylabs]

Part of #922

Scope

Prove generated literal loaders () => import(spec).then(normalizeFreshRouteModule) against upstream App.route(MaybeLazy<Route>): default/handler(s)/config/css mapping, structured error on unknown members, real bundler edges.

Design source: RFC #890 (rfc.md §5) + .llm/runs/plan-frontend-contrib--seed/design/canonical/ (rev 3).

Acceptance

• gate: normalizer maps all four members and errors on unknowns (fixtures)
• gate: route css + handlers verified end-to-end on a mounted plugin route