Add UDF virtual image builder adapted to fork stream APIs

Library/DiscUtils.Udf/DiscUtils.Udf.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <Description>Managed .NET reader for UDF optical-disc filesystems and their files, directories and extended attributes.</Description>
+    <Description>Managed .NET reader and virtual UDF 2.01 image builder for optical-disc filesystems.</Description>
 
     <PackageTags>DiscUtils;Filesystem;Optical;UDF</PackageTags>
   </PropertyGroup>

Library/DiscUtils.Udf/README.md

@@ -1,6 +1,6 @@
 # LTRData.DiscUtils.Udf
 
-Managed .NET reader for UDF optical-disc filesystems and their files, directories and extended attributes.
+Managed .NET reader and image builder for UDF optical-disc filesystems.
 
 Part of the [LTRData DiscUtils fork](https://github.com/LTRData/DiscUtils). Package IDs use `LTRData.DiscUtils`; C# namespaces remain `DiscUtils`.
 
@@ -15,13 +15,19 @@ dotnet add package LTRData.DiscUtils.Udf
 | Capability | Support |
 | --- | --- |
 | Read files and directories | Yes |
-| Create a filesystem | No |
+| Create a filesystem | UDF 2.01 with UdfBuilder |
 | Modify an existing filesystem | No |
 
 ## Usage notes
 
 UdfReader exposes files and extended attributes. Size, UsedSpace and AvailableSpace are not implemented. For automatic ISO/UDF detection, reference LTRData.DiscUtils.OpticalDisk and register its providers; this package has no generated Formats entry point.
 
+UdfBuilder creates a read-only UDF 2.01 image with one physical partition and 2048-byte logical blocks. Files are limited to 234 short allocation descriptors (approximately 234 GiB per file); the partition is limited to approximately 8 TiB. The builder preserves names, directories and file contents, but does not import source filesystem attributes, extended attributes or permissions.
+
+`Build()` and `BuildAsync(cancellationToken)` return a seekable virtual image as a `Stream`. Construction creates metadata in memory and opens no file payloads. Every image owns the streams returned by its source factories; each factory must return a new readable, seekable stream of the declared length on every call. Async image reads use the source stream's async methods. Separate builds have independent positions; a single image stream must not be read concurrently.
+
+The source-path overload checks length and last-write time whenever it opens the file. These checks do not make a content snapshot: keep source files and retained byte arrays unchanged for the image's lifetime. Unexpected truncation fails, and reads never include source data beyond a declared extent. The overloads that build to an output file or stream materialize the entire image, including payloads and zero padding.
+
 ## Example
 
 ```csharp
@@ -36,6 +42,20 @@ foreach (var file in fs.Root.GetFiles())
     Console.WriteLine(file.FullName);
 ```
 
+Create a virtual image from existing files:
+
+```csharp
+using System.IO;
+using DiscUtils.Udf;
+
+var builder = new UdfBuilder { VolumeIdentifier = "DISC" };
+builder.AddDirectory("BDMV/EMPTY");
+builder.AddFile("BDMV/index.bdmv", Path.Combine("source", "BDMV", "index.bdmv"));
+builder.AddFile("readme.txt", System.Text.Encoding.UTF8.GetBytes("Virtual UDF image"));
+using Stream image = builder.Build();
+// Serve reads/seeks from image, or materialize it with builder.Build(output).
+```
+
 ## Related packages
 
 - [LTRData.DiscUtils.OpticalDisk](https://www.nuget.org/packages/LTRData.DiscUtils.OpticalDisk)