OnBarClose / BarClosed event callback

There is no dedicated callback that fires exactly once when a bar closes. Developers currently infer bar close by comparing the previous bar index across OnCalculate calls — this is fragile, fires differently depending on chart type, and can double-fire during recalcs.

Request: add an OnBarClose(int bar) virtual method (or BarClosed event) that fires exactly once per confirmed closed bar, after all data for that bar is final. This is a standard feature in NinjaTrader and TradeStation and is essential for reliable signal generation, exit logic, and session tracking.