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.md

@@ -0,0 +1,34 @@
+---
+name: bollinger_bands
+kind: function
+lang: go
+domain: finance
+version: "1.0.0"
+purity: pure
+signature: "func BollingerBands(data []float64, period int, numStdDev float64) (upper, middle, lower []float64)"
+description: "Calcula las bandas de Bollinger (upper, middle, lower) para una serie de precios."
+tags: [finance, indicator, bollinger, bands]
+uses_functions: []
+uses_types: [bollinger_result_go_finance]
+returns: [bollinger_result_go_finance]
+returns_optional: false
+error_type: ""
+imports: [math]
+tested: false
+tests: []
+test_file_path: ""
+file_path: "functions/finance/bollinger_bands.go"
+---
+
+# bollinger_bands
+
+Calcula las bandas de Bollinger. La banda media es la SMA, y las bandas superior e inferior estan a `numStdDev` desviaciones estandar de la media. Usa desviacion estandar poblacional.
+
+## Ejemplo
+
+```go
+upper, middle, lower := finance.BollingerBands(
+    []float64{22, 24, 23, 25, 26, 28, 27, 29, 30, 28},
+    5, 2.0,
+)
+```