feat(qe-m3-01): implement GetBarsAsOf with lookahead bias prevention and complete unit tests

src/dotnet/QuantEngine.Core/Interfaces/IPriceHistoryReader.cs

@@ -0,0 +1,23 @@
+namespace QuantEngine.Core.Interfaces;
+
+/// <summary>
+/// Provides point-in-time price history access with structural lookahead-bias prevention.
+///
+/// All methods enforce: trade_date &lt;= asOfDate is guaranteed by SQL WHERE clause,
+/// not by client-side filtering. This structural guarantee prevents any code path
+/// from accidentally accessing future data relative to the computation date.
+/// </summary>
+public interface IPriceHistoryReader
+{
+    /// <summary>
+    /// Returns up to <paramref name="lookback"/> daily bars for <paramref name="ticker"/>
+    /// with trade_date &lt;= <paramref name="asOfDate"/>, ordered most-recent-first.
+    ///
+    /// GUARANTEE: Never returns a bar dated after asOfDate — enforced by SQL WHERE clause.
+    /// </summary>
+    /// <param name="ticker">Stock ticker symbol</param>
+    /// <param name="asOfDate">Observation date (inclusive upper bound)</param>
+    /// <param name="lookback">Maximum number of bars to return</param>
+    /// <returns>List of PriceHistoryDailyRecord, most-recent-first</returns>
+    Task<List<PriceHistoryDailyRecord>> GetBarsAsOf(string ticker, DateOnly asOfDate, int lookback);
+}