DXtrade: an entry never closes a hand trade, and a flatten never opens one

DXtrade dropped the intent the engine now passes, so on the live
Tradeify247 accounts an entry could still close a hand position as the
"entry" and the leg's flatten then OPEN a position with no exits. The
open/close decision moves out of vest-helpers into platform-client's
orderEffect, which both clients now follow to pick OPEN or CLOSE.

A flatten's flat read is "nothing to flatten" only when the client's own
record agrees. One empty read just after a fill the client saw may be the
gateway lagging, so it stays a plain refusal and keeps the FLATTEN FAILED
alarm. A check now fails if an engine order drops its intent or a client
stops going by orderEffect.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/platform-client.ts

@@ -171,9 +171,9 @@ export interface PlatformClient {
     /** `intent`, where the caller knows what the order is for: an entry
      *  ('open') must never close a position it didn't know was there, a
      *  flatten ('close') must never open one on an account that turned out
-     *  flat. A platform that checks it refuses the order otherwise
-     *  (NothingToCloseError for a close with nothing to close); left out, or
-     *  on a platform that doesn't check, the position decides. */
+     *  flat. A platform that checks it (orderEffect) refuses the order
+     *  otherwise (NothingToCloseError for a close with nothing to close);
+     *  left out, or on a platform that doesn't check, the position decides. */
     sendOrder(
         accountId: number,
         contractSymbol: string,
@@ -464,9 +464,9 @@ export function isOrderNotPlaced(err: unknown): boolean {
 
 /**
  * A close-only order (sendOrder's 'close') refused because a fresh read
- * showed no position in the symbol to close: the account is flat there, so a
- * flatten that gets this has nothing left to do — nothing rides without its
- * stop.
+ * showed no position in the symbol to close, and the client's own record
+ * agrees: the account is flat there, so a flatten that gets this has nothing
+ * left to do — nothing rides without its stop.
  */
 export class NothingToCloseError extends OrderNotPlacedError {
     readonly nothingToClose = true;
@@ -481,6 +481,44 @@ export function isNothingToClose(err: unknown): boolean {
     return !!err && typeof err === 'object' && (err as { nothingToClose?: unknown }).nothingToClose === true;
 }
 
+/**
+ * What an order on `side` does on a position-based platform (Vest, DXtrade),
+ * given a fresh read of the account's position in the symbol (`held`: its
+ * side, null when flat) and the client's own last record of it (`recorded`):
+ * open, or close (or reduce) — or it throws why nothing is sent. With no
+ * intent a flat account opens and the opposite side closes. An entry ('open')
+ * never closes: a hand trade the engine didn't know about would be closed in
+ * its place, and the leg's flatten would then open a position with no exits.
+ * A flatten ('close') never opens, and a flat read is NothingToCloseError only
+ * when the record agrees — one empty read just after a fill the client saw
+ * may be the gateway lagging, and a leg riding without its stop must still be
+ * shouted about. Adding to a position is never supported.
+ */
+export function orderEffect(o: {
+    symbol: string;
+    held: 'long' | 'short' | null;
+    recorded: 'long' | 'short' | null;
+    side: 'long' | 'short';
+    intent?: OrderIntent;
+}): 'open' | 'close' {
+    const { symbol, held, recorded, side, intent } = o;
+    if (held === side) {
+        throw new OrderNotPlacedError(intent === 'close'
+            ? `the account holds a ${held} ${symbol} position, not one to close — nothing sent`
+            : `adding to an open ${symbol} position is not supported`);
+    }
+    if (held && intent === 'open') {
+        throw new OrderNotPlacedError(`a ${held} ${symbol} position is already open — an entry never closes it, order not sent`);
+    }
+    if (!held && intent === 'close') {
+        if (recorded && recorded !== side) {
+            throw new OrderNotPlacedError(`a fresh read shows no ${symbol} position, but this client last saw a ${recorded} one open — nothing sent`);
+        }
+        throw new NothingToCloseError(`no ${symbol} position to close — nothing sent`);
+    }
+    return held ? 'close' : 'open';
+}
+
 /** An error's message without the rest of it. AxiosErrors carry the request
  *  config — Tradovate's login body (the password) and Bearer tokens — so they
  *  must never be logged whole. */