feat(v2): add fall event regression engine

v2/internal/spike/preprocess.go

@@ -1,43 +1,14 @@
 package spike
 
-import "fmt"
+import "silverpose/v2/internal/pose"
 
 // LetterboxBGRToNCHW converts one packed BGR frame into the RGB, CHW,
 // float32 tensor expected by the locked 640-pixel YOLO pose ONNX model.
 //
-// It intentionally uses nearest-neighbour scaling for this Spike.  T-303
-// must replace or validate this preprocessing against the V1 implementation
-// before it becomes the V2 production preprocessing path.
+// T-303 aligned the implementation with the fixed-shape Ultralytics
+// letterbox used by the V1/ONNX baseline. New V2 code should import the pose
+// package directly to retain the returned coordinate transform.
 func LetterboxBGRToNCHW(frame []byte, width, height, target int) ([]float32, error) {
-	if width <= 0 || height <= 0 || target <= 0 {
-		return nil, fmt.Errorf("frame width, height and target must be positive")
-	}
-	if len(frame) != width*height*3 {
-		return nil, fmt.Errorf("BGR frame length = %d, want %d", len(frame), width*height*3)
-	}
-
-	scaleWidth := target
-	scaleHeight := height * target / width
-	if scaleHeight > target {
-		scaleHeight = target
-		scaleWidth = width * target / height
-	}
-	padX := (target - scaleWidth) / 2
-	padY := (target - scaleHeight) / 2
-	plane := target * target
-	input := make([]float32, 3*plane)
-
-	for y := 0; y < scaleHeight; y++ {
-		sourceY := y * height / scaleHeight
-		for x := 0; x < scaleWidth; x++ {
-			sourceX := x * width / scaleWidth
-			source := (sourceY*width + sourceX) * 3
-			destination := (padY+y)*target + padX + x
-			input[destination] = float32(frame[source+2]) / 255.0
-			input[plane+destination] = float32(frame[source+1]) / 255.0
-			input[2*plane+destination] = float32(frame[source]) / 255.0
-		}
-	}
-
-	return input, nil
+	input, _, err := pose.PreprocessBGR(frame, width, height, target)
+	return input, err
 }