Pure logic test (no GPU, no model load, no upstream daVinci-MagiHuman
clone) that asserts MagiHumanSRLatentPreparationStage.forward() clears
batch.magi_static_packed_layout. Catches the f1eeb630 regression that
the existing SR-540p pipeline-parity test missed because that test
bypasses the production stage composition and re-implements the SR
denoise loop with simplified inline helpers, so the cross-stage state
transfer via ForwardBatch is never exercised.
Verified the test fails against HEAD~1 (without the f1eeb630 fix) with:
AssertionError: assert <sentinel object> is None
and passes against HEAD.