perf: 优化图片导出性能，修复测试断言问题

1. 使用 SKCodec 替代 Image.Load 读取图片元数据，跳过解码/编码
   - ExcelPicture 新增原始字节构造函数，直接写入 Excel 包
   - ExcelDrawings 新增 AddPictureFromBytes 方法
   - ExcelPackage.AddImage 修复 uri=null 时 contentType 被硬编码为 image/jpeg 的问题

2. AddPictures 支持并行加载 + 应用层去重
   - 同一 URL 只下载一次，通过 ConcurrentDictionary 缓存
   - Parallel.ForEach 并行下载，串行写入 Excel

3. AutoFit 字体缓存，优化 Linux 性能
   - ExcelRangeBase 预加载 SKTypeface 到缓存，避免每个 cell 都调用 fontconfig
   - SKTypeface 和 SKFont 在 finally 中统一释放

4. 修复测试断言问题
   - .Equals() / .Any() 返回值未断言（空操作）→ 改为 .ShouldBe() / .ShouldBeTrue()
   - AttrsExport_Test 日期格式列号修正（D2/E2 而非 E2/F2）
   - ExportWithJPG_Test 补充断言
   - AttrExportWithColAutoCenterData_Test 文件路径冲突修复
   - RequiredIf_Test Culture 全局状态泄漏修复（try/finally）
   - ImportTestColumnIndex_Test 删除调试遗留代码

5. 新增 ImageOptimization_Tests（21 个测试）
   - IdentifyImage / DecodeBase64ToBytes / ReadImageBytes 单元测试
   - AddPictureFromBytes / ExcelPicture 原始字节集成测试
   - 端到端导出测试（多图/混合/去重/Base64）

Refs #571
Refs #616（部分解决：主导出路径已用 SKCodec 替代，模板导出/导入/颜色系统仍需迁移）

Author: hueifeng

src/EPPlus/EPPlus/Drawing/ExcelDrawings.cs

@@ -352,6 +352,27 @@ public ExcelPicture AddPicture(string Name, Image image, IImageFormat format, Ur
             throw (new Exception("AddPicture: Image can't be null"));
         }
 
+        /// <summary>
+        /// 从原始字节添加图片，配合 SKCodec 读取的元数据使用
+        /// </summary>
+        public ExcelPicture AddPictureFromBytes(string name, byte[] imageBytes,
+            string contentType, int width, int height)
+        {
+            if (imageBytes == null || imageBytes.Length == 0)
+                throw new ArgumentException("imageBytes 不能为空");
+
+            if (_drawingNames.ContainsKey(name))
+                throw new Exception("Name already exists in the drawings collection");
+
+            XmlElement drawNode = CreateDrawingXml();
+            drawNode.SetAttribute("editAs", "oneCell");
+            ExcelPicture pic = new ExcelPicture(this, drawNode, imageBytes, contentType, width, height);
+            pic.Name = name;
+            _drawings.Add(pic);
+            _drawingNames.Add(name, _drawings.Count - 1);
+            return pic;
+        }
+
         /// <summary>
         /// Add a picure to the worksheet
         /// </summary>

src/EPPlus/EPPlus/Drawing/ExcelPicture.cs

@@ -166,6 +166,58 @@ private void AddNewPicture(byte[] img, string relID)
             //_drawings._pics.Add(newPic);
         }
 
+        /// <summary>
+        /// 从原始字节构造，跳过 Image.Load 和 GetImageAsByteArray
+        /// </summary>
+        internal ExcelPicture(ExcelDrawings drawings, XmlNode node,
+            byte[] imageBytes, string contentType, int width, int height)
+            : base(drawings, node, "xdr:pic/xdr:nvPicPr/xdr:cNvPr/@name")
+        {
+            XmlElement picNode = node.OwnerDocument.CreateElement("xdr", "pic", ExcelPackage.schemaSheetDrawings);
+            node.InsertAfter(picNode, node.SelectSingleNode("xdr:to", NameSpaceManager));
+            _hyperlink = null;
+            picNode.InnerXml = PicStartXml();
+
+            node.InsertAfter(node.OwnerDocument.CreateElement("xdr", "clientData", ExcelPackage.schemaSheetDrawings),
+                picNode);
+
+            var package = drawings.Worksheet._package.Package;
+            Image = null;
+            _contentType = contentType;
+            string relID = SavePictureDirectly(imageBytes, contentType);
+
+            node.SelectSingleNode("xdr:pic/xdr:blipFill/a:blip/@r:embed", NameSpaceManager).Value = relID;
+            _height = height;
+            _width = width;
+            EditAs = eEditAs.OneCell;
+            SetPixelWidth(width, 96f);
+            SetPixelHeight(height, 96f);
+            package.Flush();
+        }
+
+        private string SavePictureDirectly(byte[] imageBytes, string contentType)
+        {
+            var ii = _drawings._package.AddImage(imageBytes, null, contentType);
+
+            ImageHash = ii.Hash;
+            if (_drawings._hashes.ContainsKey(ii.Hash))
+            {
+                var relID = _drawings._hashes[ii.Hash];
+                var rel = _drawings.Part.GetRelationship(relID);
+                UriPic = UriHelper.ResolvePartUri(rel.SourceUri, rel.TargetUri);
+                return relID;
+            }
+
+            UriPic = ii.Uri;
+            ImageHash = ii.Hash;
+
+            RelPic = _drawings.Part.CreateRelationship(UriHelper.GetRelativeUri(_drawings.UriDrawing, UriPic),
+                Packaging.TargetMode.Internal, ExcelPackage.schemaRelationships + "/image");
+
+            _drawings._hashes.Add(ii.Hash, RelPic.Id);
+            return RelPic.Id;
+        }
+
         #endregion
 
         private string SavePicture(Image image, IImageFormat format)
@@ -246,7 +298,7 @@ private string PicStartXml()
         internal string ImageHash { get; set; }
 
         /// <summary>
-        /// The Image
+        /// The Image (may be null when constructed from raw bytes via AddPictureFromBytes)
         /// </summary>
         public Image Image { get; private set; }
 
@@ -256,7 +308,12 @@ private string PicStartXml()
         /// </summary>
         public IImageFormat ImageFormat { get; private set; }
 
-        internal string ContentType => ImageFormat?.DefaultMimeType;
+        /// <summary>
+        /// 从原始字节构造时，直接存储 content type 字符串
+        /// </summary>
+        private string _contentType;
+
+        internal string ContentType => _contentType ?? ImageFormat?.DefaultMimeType;
 
         /// <summary>
         /// Set the size of the image in percent from the orginal size
@@ -267,6 +324,7 @@ public override void SetSize(int Percent)
         {
             if (Image == null)
             {
+                // 原始字节构造的图片，使用已存储的 _width/_height 和标准 DPI
                 base.SetSize(Percent);
             }
             else
@@ -348,9 +406,9 @@ public override void Dispose()
         {
             base.Dispose();
             _hyperlink = null;
-            Image.Dispose();
+            Image?.Dispose();
             Image = null;
-            ImageFormat = JpegFormat.Instance;
+            _contentType = null;
         }
     }
 }

src/EPPlus/EPPlus/ExcelPackage.cs

@@ -440,8 +440,10 @@ internal ImageInfo AddImage(byte[] image, Uri uri, string contentType)
                     Packaging.ZipPackagePart imagePart;
                     if (uri == null)
                     {
-                        uri = GetNewUri(Package, "/xl/media/image{0}.jpg");
-                        imagePart = Package.CreatePart(uri, "image/jpeg", CompressionLevel.None);
+                        var effectiveContentType = string.IsNullOrEmpty(contentType) ? "image/jpeg" : contentType;
+                        var extension = GetExtensionFromContentType(effectiveContentType);
+                        uri = GetNewUri(Package, "/xl/media/image{0}" + extension);
+                        imagePart = Package.CreatePart(uri, effectiveContentType, CompressionLevel.None);
                     }
                     else
                     {
@@ -455,6 +457,23 @@ internal ImageInfo AddImage(byte[] image, Uri uri, string contentType)
             }
             return _images[hash];
         }
+
+        /// <summary>
+        /// 根据 content type 返回对应的文件扩展名
+        /// </summary>
+        private static string GetExtensionFromContentType(string contentType)
+        {
+            switch (contentType)
+            {
+                case "image/png": return ".png";
+                case "image/gif": return ".gif";
+                case "image/bmp": return ".bmp";
+                case "image/webp": return ".webp";
+                case "image/tiff": return ".tiff";
+                default: return ".jpg";
+            }
+        }
+
         internal ImageInfo LoadImage(byte[] image, Uri uri, Packaging.ZipPackagePart imagePart)
         {
 #if (Core)

src/EPPlus/EPPlus/ExcelRangeBase.cs

@@ -918,28 +918,57 @@ public void AutoFitColumns(double minimumWidth, double maximumWidth)
             }
 
             var styles = _worksheet.Workbook.Styles;
-            foreach (var cell in this)
+
+            // 字体缓存：避免每个 cell 都调用 SKTypeface.FromFamilyName（Linux 上 fontconfig 很慢）
+            var fontCache = new Dictionary<(string family, int style, float size), SKFont>();
+            var typefaces = new List<SKTypeface>();
+            var fallbackFont = new SKFont(SKTypeface.Default);
+            try
             {
-                if (_worksheet.Column(cell.Start.Column).Hidden) //Issue 15338
+                for (int fi = 0; fi < styles.Fonts.Count; fi++)
                 {
-                    continue;
-                }
+                    var excelFontXml = styles.Fonts[fi];
+                    var skFontStyle = SKFontStyle.Normal;
+                    if (excelFontXml.Bold && excelFontXml.Italic)
+                        skFontStyle = SKFontStyle.BoldItalic;
+                    else if (excelFontXml.Italic)
+                        skFontStyle = SKFontStyle.Italic;
+                    else if (excelFontXml.Bold)
+                        skFontStyle = SKFontStyle.Bold;
 
-                if (cell.Merge || cell.Style.WrapText)
-                {
-                    continue;
+                    var cacheKey = (excelFontXml.Name, skFontStyle.GetHashCode(), excelFontXml.Size);
+                    if (!fontCache.ContainsKey(cacheKey))
+                    {
+                        var fontTypeface = SKTypeface.FromFamilyName(excelFontXml.Name, skFontStyle);
+                        if (fontTypeface != null && fontTypeface.FamilyName == excelFontXml.Name)
+                        {
+                            typefaces.Add(fontTypeface);
+                            fontCache[cacheKey] = new SKFont(fontTypeface, excelFontXml.Size);
+                        }
+                    }
                 }
 
-                if (cell.Address.StartsWith("I"))
+                foreach (var cell in this)
                 {
-                }
+                    if (_worksheet.Column(cell.Start.Column).Hidden) //Issue 15338
+                    {
+                        continue;
+                    }
 
-                var ind = styles.CellXfs[cell.StyleID].Indent;
-                var textForWidth = cell.TextForWidth;
-                var t = textForWidth + (ind > 0 && !string.IsNullOrEmpty(textForWidth) ? new string('_', ind) : "");
-                if (t.Length > 32000) t = t.Substring(0, 32000); //Issue
-                using (var font = GetAvailableFont(t))
-                {
+                    if (cell.Merge || cell.Style.WrapText)
+                    {
+                        continue;
+                    }
+
+                    if (cell.Address.StartsWith("I"))
+                    {
+                    }
+
+                    var ind = styles.CellXfs[cell.StyleID].Indent;
+                    var textForWidth = cell.TextForWidth;
+                    var t = textForWidth + (ind > 0 && !string.IsNullOrEmpty(textForWidth) ? new string('_', ind) : "");
+                    if (t.Length > 32000) t = t.Substring(0, 32000); //Issue
+                    var font = GetAvailableFont(t, fontCache, fallbackFont);
                     var normalSize =
                         Convert.ToSingle(ExcelWorkbook.GetWidthPixels(font.Typeface.FamilyName, font.Size));
                     var measureWidthInPoints = font.MeasureText(t, out SKRect rect);
@@ -972,42 +1001,45 @@ public void AutoFitColumns(double minimumWidth, double maximumWidth)
                             widthInPixels > maximumWidth ? maximumWidth : widthInPixels;
                     }
                 }
-            }
 
-            _worksheet.Drawings.AdjustWidth(drawWidths);
-            _worksheet._package.DoAdjustDrawings = doAdjust;
+                _worksheet.Drawings.AdjustWidth(drawWidths);
+                _worksheet._package.DoAdjustDrawings = doAdjust;
+            }
+            finally
+            {
+                // 统一 dispose 所有缓存的字体和 typeface 对象
+                foreach (var kv in fontCache)
+                    kv.Value?.Dispose();
+                foreach (var tf in typefaces)
+                    tf?.Dispose();
+                fallbackFont?.Dispose();
+            }
         }
 
-        private SKFont GetAvailableFont(string text)
+        private SKFont GetAvailableFont(string text,
+            Dictionary<(string family, int style, float size), SKFont> fontCache,
+            SKFont fallbackFont)
         {
             var isAllAsciiSymbolOrChars =
-                !text.Any(c => c < 20 || c > 126); // use FangSong to measure the text that is not in english
+                !text.Any(c => c < 20 || c > 126);
             var styles = _worksheet.Workbook.Styles;
             for (int i = isAllAsciiSymbolOrChars ? 0 : 1; i < styles.Fonts.Count; i++)
             {
                 var skFontStyle = SKFontStyle.Normal;
                 var excelFontXml = styles.Fonts[i];
                 if (excelFontXml.Bold && excelFontXml.Italic)
-                {
                     skFontStyle = SKFontStyle.BoldItalic;
-                }
                 else if (excelFontXml.Italic)
-                {
                     skFontStyle = SKFontStyle.Italic;
-                }
                 else if (excelFontXml.Bold)
-                {
                     skFontStyle = SKFontStyle.Bold;
-                }
 
-                var fontTypeface = SKTypeface.FromFamilyName(excelFontXml.Name, skFontStyle);
-                if (fontTypeface.FamilyName == excelFontXml.Name)
-                {
-                    return new SKFont(fontTypeface, excelFontXml.Size);
-                }
+                var cacheKey = (excelFontXml.Name, skFontStyle.GetHashCode(), excelFontXml.Size);
+                if (fontCache.TryGetValue(cacheKey, out var cachedFont))
+                    return cachedFont;
             }
 
-            return new SKFont(SKTypeface.Default);
+            return fallbackFont;
         }
 
         private void SetMinWidth(double minimumWidth, int fromCol, int toCol)