Check a candidate leg plan for non-blocking warnings WITHOUT writing. Four categories: capacity (a tour's container would overflow), time-window (a leg's window does not overlap its tour), downstream-time-window (the simulated post-change schedule of an affected tour arrives at a stop after its requested window closes), and chain-deadline (a leg's projected dropoff leaves the shipment's remaining legs no chance to make the final delivery). Returns the same warnings the equivalent PUT would raise; a PUT that carries a warning is rejected with 422 unless the matching accept flag is set. Also returns tourImpacts: for each tour the plan routes a leg onto, the estimated schedule time the plan adds to that tour in seconds (driving + service + driver waiting, post-change minus current), from the same simulation that produced the warnings. With withSchedules: true the response additionally carries schedules: each affected tour's simulated before/after stop timeline, from that same simulation. Read-only — no optimistic-lock token, no state change.