feat: add portfolio snapshot read layer

trex_core/adapters/broker/paper.py

@@ -11,6 +11,7 @@ import threading
 from typing import Optional
 
 from trex_core.application.broker_port import (
+    BalanceItem,
     FillStatus,
     OrderAck,
     OrderRequest,
@@ -94,3 +95,19 @@ class PaperBroker:
                 return False
             o.cancelled = True
             return True
+
+    def get_balance(self) -> list[BalanceItem]:
+        """Deterministic per-symbol fill aggregate (REQ-031 AC-3).
+
+        Aggregates buys − sells across all filled (or partially filled)
+        paper orders, drops zero-qty rows (Design Decision 7).
+        """
+        with self._lock:
+            balances: dict[str, int] = {}
+            for o in self._orders.values():
+                if o.filled <= 0:
+                    continue
+                sign = 1 if o.side == "buy" else -1
+                balances[o.symbol] = balances.get(o.symbol, 0) + sign * int(o.filled)
+        return [BalanceItem(symbol=sym, hldg_qty=qty)
+                for sym, qty in balances.items() if qty != 0]