getCenterAverage() is null in the pose flow, so the auto DC level silently
no-op'd (composite virtual sky stayed at -4511). Derive the level from the
reference buffer render itself - perSensorImagesFromTD(gpuQuad, true)[0]
sky-half median - i.e. the exact data being zeroed, right after
setReferenceGPU uploads it. Failure-safe warnings preserved; explicit
curt.dc_center and adoption/persistence semantics unchanged.

Verify: pose run -> console 'AUTO center DC level', composite virtual sky ~0.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>