Add async I/O support for SevenZip archive initialization

[Copilot]

SevenZipArchive was performing synchronous reads during archive initialization, causing failures when used with async-only streams via OpenAsyncArchive.

Changes:

• ArchiveReader: Added OpenAsync and ReadDatabaseAsync methods that use ReadExactAsync instead of synchronous Read calls
• SevenZipArchive:
  • Added LoadFactoryAsync to initialize archive database asynchronously
  • Overrode LoadEntriesAsync to use async loading path
  • Converted LoadEntries to iterator pattern (yield return) for proper lazy evaluation

The async and sync code paths are now properly separated, with EntriesAsync using fully async I/O operations:

// Async path now works with async-only streams
await using var archive = await ArchiveFactory.OpenAsyncArchive(asyncOnlyStream);
await foreach (var entry in archive.EntriesAsync)
{
    await using var stream = await entry.OpenEntryStreamAsync();
    // Extract asynchronously
}

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