Reprice a resting limit order in place

Moves a resting limit order's price in one venue round trip. The order keeps its Kairos order_id AND its venue exchange_order_id — that is the property this endpoint exists to provide: no supersession to record, and no second order whose fill could be booked twice. The cancel-and-replace it replaces leaves the book empty of the order for the whole cancel round trip and mints a second id to reconcile. Venue support. Only venues advertising supports_native_amend in GET /exchanges/{exchange_id}/capabilities — today kalshi alone. Anything else is 409 EXCHANGE_AMEND_UNSUPPORTED, answered before the order's own state is inspected so the verdict is a permanent property of the venue rather than of this moment. There is deliberately NO internal cancel-and-replace fallback: that substitution changes queue position, order identity and the caller's failure modes, so it is the caller's decision. Reprice only. quantity is accepted solely to be refused with 409 EXCHANGE_AMEND_QUANTITY_UNSUPPORTED, before the order is read — refused rather than silently ignored so a caller never believes it resized. Limit orders only; price must be strictly between 0 and 1, in the order's own outcome's terms. This endpoint almost always returns 200. success: false is an ambiguous venue outcome, never a transport failure — the same convention POST /orders/{order_id}/cancel uses, because a 5xx invites a retry against an order that may already have been amended. It covers a terminal order, an order not yet at the venue, and a venue that refused; on every one of those the order is untouched and still resting on its old price. Branch on success, not on the status code. Queue position. Kalshi preserves it only when an amend DECREASES size; a reprice forfeits it — but so does the cancel/replace this replaces, so nothing is lost. The win is the closed off-book window, not priority. Admission. Runs the same checks a submit does — kill switch, side/kind/TIF circuit breakers evaluated against the order's own side and kind (an amend changes neither), and the order rate limit. An immediate fill from a crossing amend is booked through the normal poller path, not from this response. Auth & scope. Requires trade:execute and ownership of the order.