Report each account's last traded trade date

Sessions roll at 5 PM CT, so after tonight's open a day booked on
tomorrow's trade date is already in the day count — lastDay is what lets
the master tell those accounts apart from ones that have not traded the
current session yet.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/master-protocol.ts

@@ -40,6 +40,10 @@ export interface AccountTypeStats {
     payoutStates?: {
         days: number; profit: number; maxDay: number; sum: number; high: number;
         withdrawals?: number; target?: number; consistency?: number; minDays?: number;
+        /** Trade date (YYYY-MM-DD) of the account's most recent traded day —
+         *  after the 5 PM CT roll a day booked on tomorrow's trade date is
+         *  already counted in `days`. */
+        lastDay?: string;
     }[];
 }