feat: 15 funciones finance — indicadores, riesgo e IO de mercado

11 funciones puras con implementación real:
SMA, EMA, RSI, BollingerBands, VWAP, LogReturn, AnnualizedVolatility,
SharpeRatio, MaxDrawdown, NormalizeOHLCV, TickToOHLCV

4 funciones impuras (stubs):
FetchOHLCV, StreamTicks, WriteOHLCVToParquet, LoadOHLCVFromDuckDB

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

functions/finance/fetch_ohlcv.go

@@ -0,0 +1,9 @@
+package finance
+
+import "fmt"
+
+// FetchOHLCV obtiene datos OHLCV de un exchange para un simbolo e intervalo dados.
+// TODO: implementar conexion real al exchange.
+func FetchOHLCV(symbol, interval string) ([][]float64, error) {
+	return nil, fmt.Errorf("not implemented: FetchOHLCV(%s, %s)", symbol, interval)
+}