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

functions.go

@@ -876,4 +876,58 @@ var (
 		outNBElement *int32,
 		outReal []float64,
 	) int32
+
+	avgdev func(
+		startIdx int32,
+		endIdx int32,
+		inReal []float64,
+		optInTimePeriod int32,
+		outBegIdx *int32,
+		outNBElement *int32,
+		outReal []float64,
+	) int32
+
+	avgprice func(
+		startIdx int32,
+		endIdx int32,
+		inOpen []float64,
+		inHigh []float64,
+		inLow []float64,
+		inClose []float64,
+		outBegIdx *int32,
+		outNBElement *int32,
+		outReal []float64,
+	) int32
+
+	medprice func(
+		startIdx int32,
+		endIdx int32,
+		inHigh []float64,
+		inLow []float64,
+		outBegIdx *int32,
+		outNBElement *int32,
+		outReal []float64,
+	) int32
+
+	typprice func(
+		startIdx int32,
+		endIdx int32,
+		inHigh []float64,
+		inLow []float64,
+		inClose []float64,
+		outBegIdx *int32,
+		outNBElement *int32,
+		outReal []float64,
+	) int32
+
+	wclprice func(
+		startIdx int32,
+		endIdx int32,
+		inHigh []float64,
+		inLow []float64,
+		inClose []float64,
+		outBegIdx *int32,
+		outNBElement *int32,
+		outReal []float64,
+	) int32
 )