Add dual exit modes: fixed USDT or premium multiple.

Net PnL (after estimated close fees) drives auto close; Plan/Settings expose the choice.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/tests/test_p1_p2_rules.py

@@ -28,19 +28,47 @@ def test_signal_equal() -> None:
     assert decide(10.0, 10.0) is None
 
 
-def test_exit_premium_and_move_pct() -> None:
+def test_exit_fixed_and_premium_multiple() -> None:
+    fixed = check_exits(
+        net_pnl=15.0,
+        exit_mode="fixed_usdt",
+        net_profit_target=15,
+        premium_exit_multiple=1,
+        initial_premium=40,
+    )
+    assert fixed.reason == "fixed_usdt"
+    assert fixed.target == 15
+
     assert (
         check_exits(
-            perp_upl=50, initial_premium=40, move_pct=0.1, exit_move_pct=2
-        ).reason
-        == "premium_cover"
+            net_pnl=14.9,
+            exit_mode="fixed_usdt",
+            net_profit_target=15,
+            premium_exit_multiple=1,
+            initial_premium=40,
+        ).should_close
+        is False
     )
-    assert (
-        check_exits(
-            perp_upl=1, initial_premium=40, move_pct=2.0, exit_move_pct=2
-        ).reason
-        == "move_pct"
+
+    prem = check_exits(
+        net_pnl=40.0,
+        exit_mode="premium_multiple",
+        net_profit_target=15,
+        premium_exit_multiple=1,
+        initial_premium=40,
     )
+    assert prem.reason == "premium_multiple"
+    assert prem.target == 40
+
+    half = check_exits(
+        net_pnl=20.0,
+        exit_mode="premium_multiple",
+        net_profit_target=15,
+        premium_exit_multiple=0.5,
+        initial_premium=40,
+    )
+    assert half.should_close is True
+    assert half.target == 20
 
 
 def test_perp_pricing() -> None: