Replace the nullable unique_ptr returned by CaptureFactory::create()
with CaptureResult<std::unique_ptr<CaptureSession>> using the new
CaptureError/CaptureResult pattern, mirroring codec/error.h. The stub
now reports 'not implemented yet' as an error instead of returning
nullptr. Update handoff memory with the review outcome and
forward-looking notes for phases 3, 5, and 7.
Add a linkable CaptureFactory stub so the capture API can be consumed
without unresolved symbols. Change remaining config string_view fields to
std::string to prevent dangling references. Update handoff memory.