alpha.png varied only alpha over a constant color, so compositing atop the
average canceled L/P/Q exactly like solid.png, leaving it just as float-noise
unstable and silently dropping alpha-path coverage. Vary R/G/B with position
too so alpha.png carries real signal and reproduces byte-exactly, then
replace the blanket solid.png skip with a precise assertion on its
well-conditioned header bytes and quantized-zero scales.
Also apply a range-over-int modernizer hint in reference_test.go.
02d253b2b0