Set Price Preset

Choose the listing's price preset (#691 D1-B). Body: {kind, id, preset, params?}. preset: keep_current | gentle_decay {floor_cents?} | launch_to_target {target_cents} | major_reset {base_cents} (collections only) | custom_schedule {start_cents, target_cents, cadence_days, drop_pct} (#691 stage 2: the seller's own schedule, opt-in). For the platform presets today's price becomes the starting price; custom_schedule starts at its own start_cents. Choosing again re-anchors the schedule and resumes a paused rule. Floors and bounds are validated here. Stage 1: steps are only reported (dry run), never executed — see collector/price_rules.py.