Price a combo cash-out (SELL) without executing it

Returns the real maker-quoted proceeds for selling an open combo, and subtracts the Kairos platform fee so the preview matches what actually lands in the wallet. A client-side shares × ∏(leg price) estimate ignores the maker's spread and systematically overstates the payout — show netProceedsE6, not a local calculation. The platform-fee lookup fails closed: a tier-lookup error rejects the quote rather than displaying an optimistic zero fee. Fully-resolved and dead combos are rejected up front. No maker quotes a settled binary, so the RFQ would simply hang; a combo that has fully resolved, or that has any single RESOLVED_LOSS leg, gets an immediate 400 instead. A won, fully-resolved combo is redeemed via POST /combo/redeem, not cashed out. Rate limited at 20 quotes/minute per user (COMBO_QUOTE_RATE_LIMIT_PER_MIN), fail-closed, because each call holds an external RFQ websocket open until the gateway timeout. The 429 carries error_details.code = EXCHANGE_POLYMARKET_RATE_LIMITED. No Retry-After header. Auth & scope. Requires position:read for polymarket AND the mutation gate (it is a POST behind RequireServiceToken despite being read-only), so an API-key triple or CSRF token is needed — a bare Bearer JWT is not enough.