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/max_drawdown.md

@@ -0,0 +1,32 @@
+---
+name: max_drawdown
+kind: function
+lang: go
+domain: finance
+version: "1.0.0"
+purity: pure
+signature: "func MaxDrawdown(values []float64) (maxDD float64, start, end int)"
+description: "Calcula el maximo drawdown de una curva de equity, retornando la magnitud y los indices pico-valle."
+tags: [finance, drawdown, risk, metric]
+uses_functions: []
+uses_types: [drawdown_result_go_finance]
+returns: [drawdown_result_go_finance]
+returns_optional: false
+error_type: ""
+imports: []
+tested: false
+tests: []
+test_file_path: ""
+file_path: "functions/finance/max_drawdown.go"
+---
+
+# max_drawdown
+
+Calcula el maximo drawdown (caida maxima desde un pico hasta un valle) como fraccion del pico. Retorna la magnitud (0 a 1) y los indices de inicio y fin.
+
+## Ejemplo
+
+```go
+dd, s, e := finance.MaxDrawdown([]float64{100, 120, 90, 110, 80})
+// dd = (120-80)/120 = 0.3333, s = 1, e = 4
+```