Legs: cheapest symbol that reaches the target hold, last stands, a reserve that grows with size

- A batch sizes its leg on every mapped candidate, each at the leverage
  the broker states for it, and trades the cheapest (spread, commissions,
  expected swap) that reaches the target hold — else the quickest. The
  dashboard ranks the same way from a half-hourly candidate survey.
- Last stand: when a daily-loss stop-out would leave the account within
  15% of that loss above its max-drawdown floor (the account type's static
  max loss, or the broker's liquidation floor), the leg carries no
  daily-loss stop and is sized against the firm's own breach.
- The daily-loss reserve is 10% of the limit or 0.1% of the position,
  whichever is more, since a stop's slippage grows with the position.
- The account type's max notional is gone: the broker's margin rates cap
  each symbol. planLegSize holds the one sizing rule fireLeg, the pick and
  the dashboard share.

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

lib/platform-client.ts

@@ -18,6 +18,13 @@ export interface PlatformAccount {
     active: boolean;
 }
 
+/** How the batch's leg would size on a candidate instrument (see planLegSize):
+ *  null when it can't be sized there. */
+export interface PickContext {
+    targetHoldHours: number;
+    evaluate(contract: PlatformContract): { notional: number; holdHours: number; cost: number } | null;
+}
+
 export interface PlatformContract {
     id: number | string;
     name: string;
@@ -105,7 +112,10 @@ export interface PlatformClient {
     /** Drop the account's history from memory (the wipe routes), so it reads
      *  unloaded until the next fetch rebuilds it. */
     forgetHistory(accountId: number, accountName: string): void;
-    findFrontMonthContract(productName: string): Promise<PlatformContract | null>;
+    /** The contract a product trades as. A platform that chooses between
+     *  instruments (DXTrade's symbol map) ranks them with `pick` when given —
+     *  how a leg would size on each — and platforms that don't, ignore it. */
+    findFrontMonthContract(productName: string, pick?: PickContext): Promise<PlatformContract | null>;
     fetchInstrumentFees(symbols: string[]): Promise<{ [symbol: string]: number }>;
 
     sendOrder(