A browser grabbing the C920 kills our reader startup: the camera stays in
MJPG 1920x1080@30 (another app's format) and our OBS-refusal to re-negotiate
under SharedReadOnly contention (`SetMediaStreamPropertiesAsync` throws
'file in use / CaptureMode is SharedReadOnly') leaves it there.
CreateFrameReaderAsync(.., Bgra8) + StartAsync then refuses with
OutputFormatNotSupported — the MJPG-active source only exposes NV12 at the
reader level (clue: startup.log 19:01/19:05 sessions, same hardware that
started YUY2 640x480 fine at 08:52). Fresh launches showed no webcam and
'Add Webcam' failed.
Reader creation is now a per-candidate ladder (ReaderSubtypeCandidates):
Bgra8 for uncompressed cameras (unchanged fast path); NV12 then the
source-default for MJPG cameras — converted in OnFrameArrived like any
non-BGRA frame. Each candidate is allocated AND started under its OWN
catch: WinRT answers an unsupported subtype with a throw (E_INVALIDARG),
not a status, so a single rejected format must degrade to the next
candidate instead of aborting acquisition (creator rule — see MyMistakes
WINRT resource-allocation recipe). Rejections are logged and collected
into the final error.
Good Dog: 4 unit tests lock the candidate ordering (MJPG never Bgra8,
case-insensitive, uncompressed keeps Bgra8 first, unknown/null -> Bgra8).
301/301 green, 0 warnings. [no push]
