fix(CORE-002C): align historical units with workspace weight

tests/test_backtesting_units.py

@@ -9,9 +9,11 @@ from app.domain.backtesting_math import (
     PricePerAsset,
     asset_quantity_from_floats,
     asset_quantity_from_money,
+    asset_quantity_from_workspace_config,
     materialize_backtest_portfolio_state,
 )
 from app.domain.units import BaseCurrency, Money
+from app.models.portfolio import PortfolioConfig
 from app.services.backtesting.comparison import EventComparisonService
 from app.services.backtesting.historical_provider import SyntheticHistoricalProvider
 from app.services.event_comparison_ui import EventComparisonFixtureHistoricalPriceSource
@@ -47,6 +49,20 @@ def test_asset_quantity_from_floats_matches_workspace_backtest_conversion() -> N
     assert asset_quantity_from_floats(968000.0, 100.0, "GLD") == 9680.0
 
 
+def test_asset_quantity_from_workspace_config_uses_instrument_weight_conversion_for_gld() -> None:
+    config = PortfolioConfig(
+        gold_ounces=220.0,
+        entry_price=4400.0,
+        entry_basis_mode="weight",
+        loan_amount=222000.0,
+        margin_threshold=0.80,
+    )
+
+    quantity = asset_quantity_from_workspace_config(config, entry_spot=100.0, symbol="GLD")
+
+    assert quantity == 2200.0
+
+
 def test_materialize_backtest_portfolio_state_uses_typed_asset_boundary() -> None:
     portfolio = materialize_backtest_portfolio_state(
         symbol="GLD",