feat: add volatility and volume indicator wrappers via purego

- add TA-Lib function pointer signatures for ATR/NATR/TRANGE and AD/ADOSC/CMF/NVI/OBV/PVI/PVO in functions.go
- register new native symbols in loader.go
- add new wrapper sources: volatility_indicators.go and volume_indicators.go
- update README function matrix with new volatility and volume categories
- add ta-lib upstream as a git submodule (.gitmodules + ta-lib gitlink)

README.md

@@ -39,6 +39,12 @@ This project currently exposes a focused subset of TA-Lib functions:
 **Statistic Functions**
 - `AvgDev`, `Beta`, `Correl`, `LinearReg`, `LinearRegAngle`, `LinearRegIntercept`, `LinearRegSlope`, `StdDev`, `TSF`, `Var`
 
+**Volatility Indicators**
+- `ATR`, `NATR`, `TRANGE`
+
+**Volume Indicators**
+- `AD`, `ADOsc`, `CMF`, `NVI`, `OBV`, `PVI`, `PVO`
+
 ## Requirements
 
 - Go `1.26`