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/fetch_ohlcv.md

@@ -0,0 +1,31 @@
+---
+name: fetch_ohlcv
+kind: function
+lang: go
+domain: finance
+version: "1.0.0"
+purity: impure
+signature: "func FetchOHLCV(symbol, interval string) ([][]float64, error)"
+description: "Obtiene datos OHLCV de un exchange para un simbolo e intervalo dados."
+tags: [finance, io, exchange, fetch]
+uses_functions: []
+uses_types: [ohlcv_go_finance]
+returns: [ohlcv_go_finance]
+returns_optional: false
+error_type: "error_go_core"
+imports: [fmt]
+tested: false
+tests: []
+test_file_path: ""
+file_path: "functions/finance/fetch_ohlcv.go"
+---
+
+# fetch_ohlcv
+
+Stub para obtener datos OHLCV de un exchange. Pendiente de implementacion.
+
+## Ejemplo
+
+```go
+data, err := finance.FetchOHLCV("BTC/USDT", "1h")
+```