DXtrade: count closed-trade P&L, week-long volatility, broker leverage caps, swap in the take-profit

- Transfers: a close books its SETTLEMENT and COMMISSION under one
  transaction code, and keying by the code alone let the commission
  overwrite the P&L. Keys now carry the type, so the day-start balance,
  daily-loss room, realized P&L, daily history and books check all see
  closed-trade profit and loss.
- Volatility from a week of hourly closes (was 48 h), still measured once
  per trade date, so one quiet weekend can't triple a hold estimate.
- Each symbol's margin rate from the account's instrument details caps
  the position at available funds / rate (Tradeify: 5:1 crypto, 2:1
  stocks); the account type's max notional stays as an optional ceiling.
- Firms with overnight financing add the swap a winning leg is expected
  to pay (the winners' mean hold, (A²+2AB)/3σ²) to its take-profit, and
  the sizing and hold estimate count it.

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

lib/platform-client.ts

@@ -38,6 +38,9 @@ export interface PlatformContract {
     maxContracts?: number;
     /** Smallest order the broker takes, in engine contracts. */
     minContracts?: number;
+    /** Margin posted per dollar of notional (0.2 = 5:1 leverage), where the
+     *  broker says — caps a position at available funds ÷ this. */
+    marginRate?: number;
 }
 
 export interface PlatformClient {
@@ -45,8 +48,9 @@ export interface PlatformClient {
     /** realizedPnL is today's. pendingRoll (Tradovate): the broker still shows
      *  the ended day's record and today's share of it can't be told apart —
      *  the account waits for the broker to roll it. equity: balance plus open
-     *  P&L, where the platform reports it. */
-    accountCashBalances: { [accountId: number]: { amount: number; realizedPnL: number; pendingRoll?: boolean; equity?: number } };
+     *  P&L, where the platform reports it; availableFunds: what margin can
+     *  still draw on, where the platform reports it. */
+    accountCashBalances: { [accountId: number]: { amount: number; realizedPnL: number; pendingRoll?: boolean; equity?: number; availableFunds?: number } };
     positions: {
         [accountId: number]: {
             contractId: number | string;