Fix single-entry extraction failures for solid RAR archives

[Copilot]

Extracting a single file (for example mkisofs.exe) from a solid RAR could fail with runtime exceptions (NullReferenceException and related unpack-state failures) when using entry.WriteTo* APIs. This change hardens solid-entry extraction so direct entry extraction behaves correctly for these archives.

• Solid RAR unpack state initialization

  • Ensures UnpackV1 allocates its window buffer when processing a compressed solid entry with no pre-existing window state.
  • Removes a null-window path that could crash during back-reference copy operations.

• Targeted fallback for solid entry extraction

  • In IArchiveEntryExtensions.WriteTo / WriteToAsync, direct OpenEntryStream* extraction is still attempted first.
  • For solid entries only, known state/stream failures now retry through sequential solid-reader extraction and copy the matched entry stream to the destination.
  • Matching is done by stable entry metadata (Key, Size, IsDirectory) to avoid reference-coupling with lazy loaders.

• Regression coverage

  • Adds sync and async tests proving a non-first entry in a solid RAR can be extracted via WriteTo / WriteToAsync.

using var archive = RarArchive.OpenArchive(archivePath);
var target = archive.Entries.Where(e => !e.IsDirectory).Skip(1).First();

using var output = new MemoryStream();
target.WriteTo(output); // now succeeds for solid RAR single-entry extraction