Fix synchronous reads called in async RAR unpacker paths

[Copilot]

Async RAR extraction fails with NotSupportedException when the underlying stream is async-only (e.g., MultiVolumeReadOnlyAsyncStream), because Unpack29Async and Unpack5Async were calling sync methods that internally invoke readStream.Read(...).

Changes

• Unpack.Async.cs — Added async counterparts for three sync methods used in Unpack29Async:

  • ReadTablesAsync() — mirrors ReadTables(), replacing unpReadBuf() calls with await unpReadBufAsync(). Uses heap-allocated arrays instead of stackalloc (not permitted in async methods).
  • ReadEndOfBlockAsync() — mirrors ReadEndOfBlock(), delegates to ReadTablesAsync().
  • ReadVMCodeAsync() — mirrors ReadVMCode(), replacing unpReadBuf() with await unpReadBufAsync().
  • Updated Unpack29Async to call all three async variants.

• Unpack50.Async.cs — Updated Unpack5Async to call await ReadTablesAsync() instead of ReadTables() at both call sites.

Example (the failing path)

// Previously threw: NotSupportedException: Synchronous read is not supported in MultiVolumeReadOnlyAsyncStream
//   at Unpack.unpReadBuf()
//   at Unpack.ReadTables()        ← sync, called from async context
//   at Unpack.Unpack29Async(...)
await reader.WriteEntryToDirectoryAsync(outputPath);