Get Estimate

Pre-commit ETA range for a prospective run (v2 §4.7: authenticated on /v1, unlike the legacy route). ETA ONLY -- no cost figure exists to expose (A11 governs any future cost surface). Delegates to the browser's estimate handler (the exact run-time planner; {eta_range_sec, eta_basis, top_n}, fail-open to a null estimate, never a 500). Costs nothing quota-wise. Call before a run when the human asks for timing, and report a null range as unknown. Recover from validation_error by fixing duration_sec or top_n. For rate_limited or service_unavailable, wait retry_after_sec and retry once.