Preflight + validate a job's checks, without submitting

Run PREFLIGHT and validate a job's CHECKS without submitting anything (docs/designs/job-checks.md). Free, about two seconds, and it changes nothing: use it in a loop while you get a job spec right, because it is faster and cheaper than a rejected submission per attempt. It answers the same three things submit_job would: does the checks block parse and mean something for this shape, does the job's worst-case cost fit the remaining spend cap (a BLOCK - the arithmetic is shown), and does the node's disk fit the model this job downloads (a WARNING). A check without sufficient data stays silent rather than guessing. Pass the same parameters you would pass to submit_job; the ones that only matter to a real run (context, image, idempotency_key, mission) are not needed.