Prebuilt patched qtmultimedia FFmpeg plugin (Windows, AV1 + Speed Control)

Not a Vivace release -- a build-infrastructure artifact consumed by
release.yml/nightly.yml's CI workflow.

Supersedes qt-av1-prebuilt-win64,
which only carried the AV1 fix. This bundle carries both fixes.

Contains ffmpegmediaplugin.dll (built from Qt 6.11.1's qtmultimedia with
both patches/qtmultimedia-av1-hwaccel-disable.patch AND
patches/qtmultimedia-wsola-pitch-compensation.patch applied) plus the
matching dav1d-enabled FFmpeg DLLs it needs (avcodec-61.dll,
avformat-61.dll, avutil-59.dll, swresample-5.dll, swscale-8.dll --
soname-matched to Qt's own officially bundled FFmpeg, built from
BtbN/FFmpeg-Builds' n7.1 line).

Together these fix:

• AV1 video decoding, which the official Qt FFmpeg backend cannot do
  reliably -- see the "AV1 support" section in README.md.
• Audio pitch-compensation artifacts at non-1x playback speed (vibrato/
  echo speeding up, buzz/strips slowing down) -- see the "Audio
  speed/pitch compensation" section in README.md.

Rebuilt locally against a Qt 6.11.1 msvc2022_64 kit; update this
release's asset whenever either patch or the Qt version changes.