New Windows facade on Session (window list, current frame, topmost
window in z-order, owning-app activation) and push-based per-window
capture streams delivering zero-copy VideoFrame values with the
density/content-rect metadata needed to map frame pixels back to
desktop coordinates (toWindowPoint is the single implementation of
that formula; robot/mac/VideoFrame.h is the typed buffer bridge).

Streams guarantee serialized, ordered callbacks and a drain on stop():
when stop() returns no callback is executing or will start. macOS only
(ScreenCaptureKit); other platform builds report Unsupported through
the new canEnumerateWindows/canStreamWindows capabilities.

- handle streamDidBecomeInactive so a closed captured window reports a
  stream death instead of leaving isRunning() true forever
- decide death reports synchronously on the delegate thread: a death
  that precedes a concurrent stop() can no longer be suppressed by the
  async hop racing the stop flag (delivery stays serialized and inside
  the stop() drain)
- guard startStream behind macOS 14 (pointPixelScale/contentRect) and
  report Unsupported below it instead of crashing at first use
- validate WindowStreamOptions.maxFps at the facade (finite, 1-1000)
  before the int32 timescale conversion
- derive frame age from SCStreamFrameInfoDisplayTime (mach absolute
  ticks, same clock as mach_absolute_time) with the PTS-vs-host-clock
  math as fallback
- make CoreVideo a PUBLIC link dependency: the installed robot/mac
  bridge hands consumers a CVPixelBufferRef they lock via CoreVideo
- widen the build-directory ignore to build*/