Explorer can consume FILECONTENTS streams with reads much smaller than
the 1 MiB protocol limit. Previously each IStream::Read became a
synchronous cross-VM request, including client-side path validation and
a fresh file open.
Give each clipboard file stream a lazy 1 MiB absolute-offset read-ahead
window. Serve sequential reads and seeks within that window locally.
Bypass the cache for 1 MiB-or-larger reads and for CopyTo so bulk
callers avoid an extra copy. Preserve EOF, partial-read, failure, seek,
and clone semantics.
bf8e68c888