Prefix fallback root query keys in adapter outputs
Adapter routing forwards a root parameter such as lang as nxtPlang. The
servable-fallback branch used the unprefixed name in config.allowQuery,
unlike the existing blocking branch. That mismatch can discard the value
before the request reaches the function or partitions its cache entry.
Prefix the root names consistently. Add a production fixture that runs
the real emitter against real build artifacts and a servable-root-fallback
manifest input. Current inference emits blocking root entries, so the
test supplies that one input explicitly rather than depending on the new
paramMatching API. Its expected nxtPlang fails against the old lang output.
This changes only Next.js output and requires no adapter implementation
change. The contract test is not a live deployment/function-grouping test.
