Report each firm's trading mix: open positions and today's entries by instrument

The master knew how many accounts were in a trade, not in what. Each firm's
report now carries its open positions by instrument (accounts long and
short, contracts held) and today's entries the same way.

A position's contract is named by its instrument: DXtrade's carries the
symbol; a futures one is looked up among the contracts this machine has
entered (recorded at the fill and kept), then the resolver's cache and
TopstepX's own. Entries are counted as they fill, per trade date, in a
setting, so a restart doesn't lose the day.

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

lib/topstepx-class.ts

@@ -1758,6 +1758,12 @@ export class TopstepXClient implements PlatformClient {
 
     // ── lifecycle ────────────────────────────────────────────────────────────
 
+    /** Contract id → the root it was resolved for, for the reporter's trade
+     *  mix (lib/trade-mix.ts). */
+    public contractRoots(): Map<string, string> {
+        return new Map([...this.contractByProduct].map(([root, { contract }]) => [String(contract.id), root]));
+    }
+
     /** The session token, for the live quote feed's market hub
      *  (lib/live-quotes.ts); null while logged out. */
     public quoteToken(): string | null {