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/annualized_volatility.go

@@ -0,0 +1,25 @@
+package finance
+
+import "math"
+
+// AnnualizedVolatility calcula la volatilidad anualizada a partir de una serie de retornos.
+// periodsPerYear indica cuantos periodos hay en un anio (e.g. 252 para retornos diarios).
+// Retorna stddev(returns) * sqrt(periodsPerYear).
+func AnnualizedVolatility(returns []float64, periodsPerYear float64) float64 {
+	n := len(returns)
+	if n < 2 {
+		return 0
+	}
+	var sum float64
+	for _, r := range returns {
+		sum += r
+	}
+	mean := sum / float64(n)
+	var variance float64
+	for _, r := range returns {
+		diff := r - mean
+		variance += diff * diff
+	}
+	variance /= float64(n - 1)
+	return math.Sqrt(variance) * math.Sqrt(periodsPerYear)
+}