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/ema.md

@@ -0,0 +1,33 @@
+---
+name: ema
+kind: function
+lang: go
+domain: finance
+version: "1.0.0"
+purity: pure
+signature: "func EMA(data []float64, period int) []float64"
+description: "Calcula la media movil exponencial (EMA) sobre una serie de datos con un periodo dado."
+tags: [finance, indicator, ema, moving-average]
+uses_functions: []
+uses_types: []
+returns: []
+returns_optional: false
+error_type: ""
+imports: []
+tested: false
+tests: []
+test_file_path: ""
+file_path: "functions/finance/ema.go"
+---
+
+# ema
+
+Calcula la media movil exponencial (Exponential Moving Average). Se inicializa con la SMA de los primeros `period` elementos. El multiplicador es `2 / (period + 1)`.
+
+## Ejemplo
+
+```go
+result := finance.EMA([]float64{10, 11, 12, 13, 14, 15}, 3)
+// result[2] = SMA de los primeros 3 = 11.0
+// result[3] en adelante usa suavizado exponencial
+```