Recording (macOS): capture and tag BT.709 colour #943

Part of #920 (rank 16). Effort S, risk low.

Problem

• The compositor decodes every recording as BT.709 studio range. Windows (fix(windows): encode H.264 High with the BT.709 colour the compositor expects #929) and Linux (fix(linux): convert and tag the PipeWire helper's video as BT.709 #932) were found writing BT.601; macOS has never been measured.
• ScreenCaptureKit captures kCVPixelFormatType_32BGRA with no colorSpaceName, and the writer sets no AVVideoColorPropertiesKey, so VideoToolbox picks the matrix and the tags (ScreenCaptureRecorder.swift). A P3 display adds a gamut question on top.
• BGRA also costs 4 bytes a pixel with queueDepth = 6: about 200 MB of buffers at 4K.

Proposal

• Ask ScreenCaptureKit for sRGB (colorSpaceName) and set AVVideoColorPropertiesKey to BT.709 primaries, transfer and matrix on the writer.
• If it holds up, capture kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange with a BT.709 colorMatrix, which also divides buffer memory by about 2.7.

Verify

• On a Mac: ffprobe -show_streams | grep color_ reads tv / bt709 / bt709 / bt709; solid red read back as raw YUV gives Y≈63, Cb≈102, Cr≈240.