新增边缘阴影清除：从四边向内扫描暗像素区域推白

除法归一化对均匀阴影免疫，但边缘深色阴影的bg也被拉低，
除法后比值偏低变灰色。在最终LUT之后加入边缘扫描：
从上下左右四边向内逐列/行扫描原始灰度，低于纸面亮度60%
的连续暗像素设为纯白，遇到亮像素停止。最大扫描深度1/4。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CamScanner.cs

@@ -418,6 +418,77 @@ public static class DocumentScanner
         whiteLutMat.Dispose();
         sharpened.Dispose();
 
+        // --- g: 边缘阴影清除 ---
+        // 从四边向内扫描原始灰度图，连续暗像素区域标记为阴影→纯白
+        // 除法归一化对均匀阴影免疫，但边缘深色阴影bg也被拉低，
+        // 除法后比值偏低变成灰色，需要额外清除
+        int cx1 = w / 4;
+        int cx2 = w * 3 / 4;
+        int cy1 = h / 4;
+        int cy2 = h * 3 / 4;
+        long centerSum = 0;
+        int centerCount = 0;
+        for (int py = cy1; py < cy2; py++)
+        {
+            for (int px = cx1; px < cx2; px++)
+            {
+                centerSum += grayData[py * w + px];
+                centerCount++;
+            }
+        }
+        int paperBright = (int)(centerSum / Math.Max(centerCount, 1));
+        int darkThresh = (int)(paperBright * 0.6);
+        int maxScanDepth = Math.Max(w, h) / 4; // 最多扫描1/4深度
+
+        byte[] cleanedData = new byte[w * h];
+        Marshal.Copy(cleaned.Data, cleanedData, 0, cleanedData.Length);
+
+        // 上边缘
+        for (int x = 0; x < w; x++)
+        {
+            for (int y = 0; y < Math.Min(maxScanDepth, h); y++)
+            {
+                if (grayData[y * w + x] < darkThresh)
+                    cleanedData[y * w + x] = 255;
+                else
+                    break;
+            }
+        }
+        // 下边缘
+        for (int x = 0; x < w; x++)
+        {
+            for (int y = h - 1; y >= Math.Max(0, h - maxScanDepth); y--)
+            {
+                if (grayData[y * w + x] < darkThresh)
+                    cleanedData[y * w + x] = 255;
+                else
+                    break;
+            }
+        }
+        // 左边缘
+        for (int py = 0; py < h; py++)
+        {
+            for (int x = 0; x < Math.Min(maxScanDepth, w); x++)
+            {
+                if (grayData[py * w + x] < darkThresh)
+                    cleanedData[py * w + x] = 255;
+                else
+                    break;
+            }
+        }
+        // 右边缘
+        for (int py = 0; py < h; py++)
+        {
+            for (int x = w - 1; x >= Math.Max(0, w - maxScanDepth); x--)
+            {
+                if (grayData[py * w + x] < darkThresh)
+                    cleanedData[py * w + x] = 255;
+                else
+                    break;
+            }
+        }
+        Marshal.Copy(cleanedData, 0, cleaned.Data, cleanedData.Length);
+
         // 转回3通道
         Mat output = new Mat();
         Cv2.CvtColor(cleaned, output, ColorConversionCodes.GRAY2BGR);