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/bollinger_bands.go

@@ -0,0 +1,32 @@
+package finance
+
+import "math"
+
+// BollingerBands calcula las bandas de Bollinger: upper, middle (SMA), lower.
+// Los primeros period-1 elementos de cada slice son 0.
+func BollingerBands(data []float64, period int, numStdDev float64) (upper, middle, lower []float64) {
+	n := len(data)
+	upper = make([]float64, n)
+	middle = make([]float64, n)
+	lower = make([]float64, n)
+	if period <= 0 || period > n {
+		return
+	}
+	for i := period - 1; i < n; i++ {
+		var sum float64
+		for j := i - period + 1; j <= i; j++ {
+			sum += data[j]
+		}
+		mean := sum / float64(period)
+		var variance float64
+		for j := i - period + 1; j <= i; j++ {
+			diff := data[j] - mean
+			variance += diff * diff
+		}
+		std := math.Sqrt(variance / float64(period))
+		middle[i] = mean
+		upper[i] = mean + numStdDev*std
+		lower[i] = mean - numStdDev*std
+	}
+	return
+}