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/vwap.go

@@ -0,0 +1,19 @@
+package finance
+
+// VWAP calcula el Volume Weighted Average Price.
+// prices y volumes deben tener la misma longitud.
+// Retorna 0 si el volumen total es 0 o los slices estan vacios.
+func VWAP(prices, volumes []float64) float64 {
+	if len(prices) == 0 || len(prices) != len(volumes) {
+		return 0
+	}
+	var cumPV, cumV float64
+	for i := range prices {
+		cumPV += prices[i] * volumes[i]
+		cumV += volumes[i]
+	}
+	if cumV == 0 {
+		return 0
+	}
+	return cumPV / cumV
+}