`device.activeFormat.formatDescription` reports the device's
*selected* format, but on macOS the session-preset remap means the
data output sometimes delivers a different resolution than what
the active format claims. The mismatch surfaced as a stretched
preview on macOS: camera_thumb's SizedBox was sized for a 4:3
buffer, the FittedBox(cover) was given a 16:9 texture, so the
texture stretched to fill the wrongly-shaped box.
PreviewSink now snapshots `CVPixelBufferGet{Width,Height}` from
the first sample buffer that arrives and forwards it via a
callback. CameraInstance hooks the callback to emit the existing
`previewSizeChanged` event (same shape the Android backend uses).
The Dart controller writes it into `value.previewSize`,
camera_thumb's SizedBox snaps to the real buffer aspect, and
FittedBox(cover) crops cleanly without stretching.
Identical wire shape and event name as the Android equivalent —
no Dart changes needed.
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