feat: 投影法波谷检测+白底硬切断+帧底部对齐+gifmaker透明GIF

- prompt_agent: 精灵表/瓦片集间隙从2-4px放宽到8-16px纯白
- splitsprite removeWhiteBg: dist<threshold/2直接A=0，消除半透明残留
- splitsprite findCuts: 波峰-波谷检测替代死阈值，归并原阈值回退
- splitsprite alignCenter: 全局画布底部对齐替代独立居中，人物不跳帧
- splitsprite MinFillRatio 调至0.14，MinGapWidth调至2
- 新增 pkg/gifmaker: 统一画布+透明索引0+DisposalBackground
- test_prompt_to_gif 改用gifmaker包

backend/pkg/gifmaker/gifmaker.go

@@ -0,0 +1,128 @@
+// Package gifmaker encodes sprite animation frames into a GIF preview.
+//
+// Features:
+//   - Unified canvas: all frames normalized to the same dimensions
+//   - Transparent background: palette index 0 = fully transparent
+//   - DisposalBackground: each frame clears the previous one, no ghosting
+//
+// Pipeline integration:
+//
+//	frames, _ := splitsprite.Process(img, splitsprite.DefaultOptions())
+//	gifmaker.Save("preview.gif", frames, nil)
+package gifmaker
+
+import (
+	"fmt"
+	"image"
+	"image/color"
+	"image/gif"
+	"io"
+	"os"
+)
+
+// Options configures GIF generation.
+type Options struct {
+	// Delay is the frame delay in 1/100s (default 10).
+	Delay int
+	// MaxColors is the maximum palette size (default 256).
+	MaxColors int
+}
+
+// DefaultOptions returns sensible defaults.
+func DefaultOptions() *Options {
+	return &Options{
+		Delay:     10,
+		MaxColors: 256,
+	}
+}
+
+// Save is a convenience wrapper that writes frames to a GIF file.
+func Save(path string, frames []image.Image, opts *Options) error {
+	f, err := os.Create(path)
+	if err != nil {
+		return fmt.Errorf("create gif file: %w", err)
+	}
+	defer f.Close()
+	return Encode(f, frames, opts)
+}
+
+// Encode writes an animated GIF to w. All frames are normalized to a unified
+// canvas (max width/height across frames), the palette starts with a
+// transparent color, and DisposalBackground prevents inter-frame ghosting.
+func Encode(w io.Writer, frames []image.Image, opts *Options) error {
+	if len(frames) == 0 {
+		return fmt.Errorf("no frames to encode")
+	}
+	if opts == nil {
+		opts = DefaultOptions()
+	}
+	delay := opts.Delay
+	if delay <= 0 {
+		delay = 10
+	}
+	maxColors := opts.MaxColors
+	if maxColors <= 0 || maxColors > 256 {
+		maxColors = 256
+	}
+
+	// Unified canvas
+	maxW, maxH := 0, 0
+	for _, f := range frames {
+		b := f.Bounds()
+		if b.Dx() > maxW {
+			maxW = b.Dx()
+		}
+		if b.Dy() > maxH {
+			maxH = b.Dy()
+		}
+	}
+
+	// Palette with transparent at index 0
+	pal := color.Palette{color.RGBA{0, 0, 0, 0}}
+	seen := make(map[color.RGBA]bool)
+	for _, fr := range frames {
+		b := fr.Bounds()
+		for y := b.Min.Y; y < b.Max.Y; y += 3 {
+			for x := b.Min.X; x < b.Max.X; x += 3 {
+				r, g, bl, a := fr.At(x, y).RGBA()
+				c := color.RGBA{uint8(r >> 8), uint8(g >> 8), uint8(bl >> 8), uint8(a >> 8)}
+				if !seen[c] && len(pal) < maxColors-1 {
+					seen[c] = true
+					pal = append(pal, c)
+				}
+			}
+		}
+	}
+
+	anim := &gif.GIF{
+		Config: image.Config{Width: maxW, Height: maxH},
+	}
+
+	for _, frame := range frames {
+		pl := image.NewPaletted(image.Rect(0, 0, maxW, maxH), pal)
+		// Manually map pixels: transparent → index 0, colored → nearest palette
+		b := frame.Bounds()
+		for y := 0; y < maxH; y++ {
+			for x := 0; x < maxW; x++ {
+				sx := x + b.Min.X
+				sy := y + b.Min.Y
+				if sx < b.Max.X && sy < b.Max.Y {
+					r, g, bl, a := frame.At(sx, sy).RGBA()
+					if a > 0 {
+						c := color.RGBA{uint8(r >> 8), uint8(g >> 8), uint8(bl >> 8), uint8(a >> 8)}
+						pl.Set(x, y, c)
+					}
+					// else: stays at index 0 (transparent)
+				}
+			}
+		}
+
+		anim.Image = append(anim.Image, pl)
+		anim.Delay = append(anim.Delay, delay)
+		anim.Disposal = append(anim.Disposal, gif.DisposalBackground)
+	}
+
+	anim.LoopCount = 0
+	anim.BackgroundIndex = 0
+	return gif.EncodeAll(w, anim)
+}