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/sharpe_ratio.md

@@ -0,0 +1,31 @@
+---
+name: sharpe_ratio
+kind: function
+lang: go
+domain: finance
+version: "1.0.0"
+purity: pure
+signature: "func SharpeRatio(returns []float64, riskFreeRate float64, periodsPerYear float64) float64"
+description: "Calcula el ratio de Sharpe anualizado a partir de retornos, tasa libre de riesgo y frecuencia."
+tags: [finance, sharpe, risk, ratio]
+uses_functions: []
+uses_types: []
+returns: []
+returns_optional: false
+error_type: ""
+imports: [math]
+tested: false
+tests: []
+test_file_path: ""
+file_path: "functions/finance/sharpe_ratio.go"
+---
+
+# sharpe_ratio
+
+Calcula el ratio de Sharpe: `(mean(returns) - riskFreeRate) / stddev(returns) * sqrt(periodsPerYear)`. Usa desviacion estandar muestral.
+
+## Ejemplo
+
+```go
+sr := finance.SharpeRatio([]float64{0.01, 0.02, -0.005, 0.015, 0.008}, 0.0001, 252)
+```