KiteVideoUploadProfiler

The measurement instrument: the desktop upload path's cost per published frame, kept as raw samples so a percentile can be taken and split so the expensive half is named rather than guessed.

KiteVideoFrameCost stays the authority for the sample count, the mean and the worst; it holds four numbers by design and no distribution. This records the SAME window, the convert plus the Skia image build, and costs one volatile read per frame while no run is active.

Desktop only, and deliberately global: the converter and the image pool are built inside KiteVideoState, so a measurement has no other seam to reach them through.