Add UDF virtual image builder adapted to fork stream APIs

[Olof-Lagerkvist]

Add a seekable, read-only UDF 2.01 image builder from DiscUtils/DiscUtils#312. Build() creates metadata in memory and opens file payloads only when the resulting image is read, allowing existing folders to be exposed as an image without duplicating their file contents.

The first commit imports upstream commit 48a7951a7f2398d21586e09d74dc089e0bc2a071 with an upstream source reference and author credit in a Co-authored-by trailer. The second adapts it to this fork using the migration guide and the current source contracts:

• Use Stream for build results and BuilderBufferExtent for metadata.
• Implement the extent's required span reads and Dispose(bool) lifecycle.
• Delegate memory/array async reads to the source stream's async API, forward cancellation, and open file sources with async support.
• Preserve declared extent boundaries across every read API and dispose sources if validation throws.
• Use span-based endian writes, nullable annotations, file-scoped namespaces, and enumerable directory results in tests.
• Document creation support, source ownership/stability, materialization behavior, and format limits in the package README, package description, and capability matrix.

The upstream image layout is retained: one physical partition, 2048-byte blocks, short allocation descriptors, approximately 234 GiB per file and 8 TiB per partition. Existing UDF filesystem editing is not added.

Validation:

• UDF project builds on net46, net48, netstandard2.0, netstandard2.1, net8.0, net9.0, and net10.0: zero warnings/errors.
• 21 focused tests pass on .NET 10: seven imported UDF tests, thirteen new API/lifetime test cases, and the existing BuiltStream test.
• Checks include an 81 GiB synthetic file, Unicode/tree round trips, multi-block directories, independent builds, real async method dispatch/materialization, cancellation, source mutation/truncation, extent padding, and disposal.
• The full runtime test suite and upstream's native libbluray/HTTP-range scenario were not rerun.

Prepared for owner review; no merge or auto-merge is requested.