Polymarket closed (realized-PnL) positions for a user (Data API)

Verbatim passthrough of the official Polymarket data-api `/closed-positions` — realized-PnL rows for a wallet's CLOSED positions (asset / conditionId / outcomeIndex / curPrice / realizedPnl / timestamp / title / …), returned as `native`. Sibling of `data/positions`; the upstream FIFO/fee accounting is authoritative, so Artery never reshapes. Two usual call shapes: `?user=&limit=&sortBy=` (closed list) and `?user=&market=<conditionId>` (single closed position for a share image). Short server cache (~10s) — a just-closed position may lag the realtime fills stream by up to the TTL. QF-10938 #2.