feat(overlap): add advanced overlap studies wrappers and wire TA-Lib symbols

- add TA-Lib function pointer signatures in functions.go for:
  MA, MAMA, MAVP, MIDPOINT, MIDPRICE, SAR, SAREXT, SMA, T3, TEMA, TRIMA, WMA
- register new symbols in Load() via purego.RegisterLibFunc in loader.go
- add public overlap study wrappers in overlap_studies.go with consistent nil-on-failure behavior
- update README overlap studies list to match implemented API

README.md

@@ -31,7 +31,7 @@ This project currently exposes a focused subset of TA-Lib functions:
 - Stochastic family: `Stoch`, `StochF`, `StochRSI`
 
 **Overlap Studies**
-- `AccBands`, `BBands`, `DEMA`, `EMA`, `HT_TRENDLINE`, `KAMA`
+- `AccBands`, `BBands`, `DEMA`, `EMA`, `HT_TRENDLINE`, `KAMA`, `MA`, `MAMA`, `MAVP`, `MidPoint`, `MidPrice`, `SAR`, `SARExt`, `SMA`, `T3`, `TEMA`, `TRIMA`, `WMA`
 
 ## Requirements