feat(backtest): Order/Position/Trade dataclasses with fees

Side StrEnum (long/short/flat), frozen dataclasses con calcolo
unrealized_pnl per Position e gross/fees/net_pnl per Trade
(fees in basis point, default 5bp). 4 test TDD passing,
ruff + mypy strict clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit/test_backtest_orders.py

@@ -0,0 +1,38 @@
+from datetime import UTC, datetime
+
+import pytest
+
+from multi_swarm.backtest.orders import Order, Position, Side, Trade
+
+
+def test_order_validates_side() -> None:
+    o = Order(ts=datetime(2024, 1, 1, tzinfo=UTC), side=Side.LONG, size=1.0)
+    assert o.side == Side.LONG
+
+
+def test_position_pnl_long() -> None:
+    pos = Position(side=Side.LONG, entry_price=100.0, size=2.0)
+    assert pos.unrealized_pnl(110.0) == pytest.approx(20.0)
+    assert pos.unrealized_pnl(90.0) == pytest.approx(-20.0)
+
+
+def test_position_pnl_short() -> None:
+    pos = Position(side=Side.SHORT, entry_price=100.0, size=2.0)
+    assert pos.unrealized_pnl(110.0) == pytest.approx(-20.0)
+    assert pos.unrealized_pnl(90.0) == pytest.approx(20.0)
+
+
+def test_trade_realized_pnl_with_fees() -> None:
+    t = Trade(
+        entry_ts=datetime(2024, 1, 1, tzinfo=UTC),
+        exit_ts=datetime(2024, 1, 2, tzinfo=UTC),
+        side=Side.LONG,
+        size=1.0,
+        entry_price=100.0,
+        exit_price=110.0,
+        fees_bp=5.0,
+    )
+    # gross 10, fees = 5bp * (100+110) = 0.0005 * 210 = 0.105
+    assert t.gross_pnl == pytest.approx(10.0)
+    assert t.fees == pytest.approx(0.105)
+    assert t.net_pnl == pytest.approx(9.895)