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Return the compiled instance from Grape::API::Instance.compile! (#2835)

.call and .recognize_path compiled the API and then read @instance again
to serve the request:

    def call(env)
      compile!
      @instance.call(env)
    end

change! (which `helpers` and `mount` invoke on a live API) sets @instance
to nil, so a change! landing between the two reads leaves both methods
calling nil -- NoMethodError: undefined method 'call' for nil.

Have compile! hand back the instance it compiled and let both callers use
that value, so the read is atomic. compile! previously returned nil when
the API was already compiled, so always returning the instance is a
strict superset of its old contract.

The window is small in practice -- a request thread spends nearly all of
its time inside `new` when recompiling -- so this is a latent race rather
than one reproduced under natural contention; the specs drive change!
into the window deterministically.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

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