fix(windows): keep SDR matrix through HDR output transition

[zortos293]

A live Windows HEVC/P010 session decoded about 60 FPS for 40 seconds, then Media Foundation switched its output transfer to SDR and primaries to BT.709 while leaving MF_MT_YUV_MATRIX absent. OpenNOW inherited the original HDR BT.2020 matrix, rejected the mixed format, and restarted the decoder every ~250 ms, leaving black video and an apparent 4 FPS. A diagnostic-only first push confirmed negotiated=Pq/Bt2020/Bt2020 decoded=Sdr/Bt709/Bt2020 explicit=true/true/false on the affected machine.

When output explicitly becomes SDR/BT.709 and omits its matrix, choose the BT.709 matrix rather than carrying the original HDR one. Explicit contradictory matrix values still fail validation, and PQ/HLG transitions retain their BT.2020 requirements. This matches Media Foundation’s documented treatment of the unknown matrix as BT.709. A Windows-only regression test covers absent and unknown matrix values and the contradictory explicit case. Color-only values are retained in failure diagnostics without logging session data.

Full streamer workspace tests, strict Clippy, formatting and diff checks passed locally. Windows CI and a repeat live HDR session on the affected PC are pending before the draft is ready.

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