behavior change: mpv now uses explicit cache=yes, cache_pause=no
(stutter over pause for short clips), 50MiB demuxer buffer cap,
20s read-ahead, and 10s network timeout (down from ~60s default).
Improves first-frame latency on uncached video streams and surfaces
stalled-connection errors faster.
Step 4 of the gui/app.py + gui/preview.py structural refactor. Pure
move: the OpenGL render-context host and its concrete QOpenGLWidget
companion are now in their own module under media/. The mid-file
`from PySide6.QtOpenGLWidgets import QOpenGLWidget as _QOpenGLWidget`
import that used to sit between the two classes moves with them to
the new module's import header. preview.py grows another re-export
shim line so VideoPlayer (still in preview.py) can keep constructing
_MpvGLWidget unchanged. Shim removed in commit 14.
