Start a Monte Carlo job (metered as one run; the first slice runs now)

Scope: solve. Validates and caps the design like POST /v1/designs/monte-carlo (the trial cap is the plan's; the CPU budget is 5,000,000 node-trials rather than 120,000 — meta.clamped says if either applied), charges ONE Monte Carlo run, then runs about 3 s of trials inside this request (allow up to 5 s: the time is checked between trials, and this first slice also validates the design). If that finished the job the response is 200 with result — the synchronous summary with nodeStats (per-node min / mean / max / stddev) in place of the per-trial array — otherwise 202 with the job's status and progress; poll GET /v1/jobs/{id}. Two active jobs per account (429 TOO_MANY_JOBS). Idempotency-Key is honoured: a replay returns the same job. Pro and Team only: a Free account is refused with 403 PLAN_REQUIRED (one sentence and the trial link).