feat(price-transform): add price transform and statistic function wrappers

New file price_transform.go implements five new public wrappers:
- AvgPrice  (TA_AVGPRICE)  – mean of OHLC per bar
- MedPrice  (TA_MEDPRICE)  – (High + Low) / 2
- TypPrice  (TA_TYPPRICE)  – (High + Low + Close) / 3
- WCLPrice  (TA_WCLPRICE)  – (High + Low + 2×Close) / 4
- AvgDev    (TA_AVGDEV)    – rolling average absolute deviation from SMA

- add corresponding low-level function pointer signatures in functions.go
- register all five symbols in Load() in loader.go
- update README.md with Price Transforms and Statistic Functions sections

README.md

@@ -33,6 +33,12 @@ This project currently exposes a focused subset of TA-Lib functions:
 **Overlap Studies**
 - `AccBands`, `BBands`, `DEMA`, `EMA`, `HT_TRENDLINE`, `KAMA`, `MA`, `MAMA`, `MAVP`, `MidPoint`, `MidPrice`, `SAR`, `SARExt`, `SMA`, `T3`, `TEMA`, `TRIMA`, `WMA`
 
+**Price Transforms**
+- `AvgPrice`, `MedPrice`, `TypPrice`, `WCLPrice`
+
+**Statistic Functions**
+- `AvgDev`
+
 ## Requirements
 
 - Go `1.26`