Handle ulong values for EXIF Long writes

Fixes EXIF serialization when `ExifLong8Array` downgrades to `ExifDataType.Long` but still stores values as `ulong`. `ExifWriter.WriteValue` now handles `ulong` inputs in the `Long` branch by writing 32-bit values, and a BigTIFF metadata test verifies mixed in-range values are written correctly as two little-endian `uint`s.

src/ImageSharp/Metadata/Profiles/Exif/ExifWriter.cs

@@ -403,6 +403,13 @@ internal sealed class ExifWriter
                     return WriteUInt32((uint)longNumber, destination, offset);
                 }
 
+                // ExifLong8Array retains ulong storage but reports Long when every value fits
+                // in 32 bits, allowing BigTIFF offsets to be serialized by classic EXIF writers.
+                if (value is ulong long8Value)
+                {
+                    return WriteUInt32((uint)long8Value, destination, offset);
+                }
+
                 return WriteUInt32((uint)value, destination, offset);
             case ExifDataType.Long8:
                 return WriteUInt64((ulong)value, destination, offset);

tests/ImageSharp.Tests/Formats/Tiff/BigTiffMetadataTests.cs

@@ -88,6 +88,21 @@ public class BigTiffMetadataTests
         Assert.Equal(ExifDataType.Long8, long8.DataType);
     }
 
+    [Fact]
+    public void ExifLong8Array_CanWriteValuesAsLong()
+    {
+        ExifLong8Array long8 = new(ExifTagValue.StripOffsets);
+        Assert.True(long8.TrySetValue(new long[] { 1, uint.MaxValue }));
+        Assert.Equal(ExifDataType.Long, long8.DataType);
+
+        byte[] buffer = new byte[8];
+        int written = ExifWriter.WriteValue(long8, buffer, 0);
+
+        Assert.Equal(buffer.Length, written);
+        Assert.Equal(1U, BinaryPrimitives.ReadUInt32LittleEndian(buffer));
+        Assert.Equal(uint.MaxValue, BinaryPrimitives.ReadUInt32LittleEndian(buffer.AsSpan(4)));
+    }
+
     [Fact]
     public void ExifSignedLong8Array()
     {