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

loader.go

@@ -114,6 +114,12 @@ func Load() (uintptr, error) {
 	purego.RegisterLibFunc(&trima, ptr, "TA_TRIMA")
 	purego.RegisterLibFunc(&wma, ptr, "TA_WMA")
 
+	purego.RegisterLibFunc(&avgdev, ptr, "TA_AVGDEV")
+	purego.RegisterLibFunc(&avgprice, ptr, "TA_AVGPRICE")
+	purego.RegisterLibFunc(&medprice, ptr, "TA_MEDPRICE")
+	purego.RegisterLibFunc(&typprice, ptr, "TA_TYPPRICE")
+	purego.RegisterLibFunc(&wclprice, ptr, "TA_WCLPRICE")
+
 	return ptr, nil
 }