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4b3a861303
The 500-image randomized differential filled every byte randomly, so hasAlpha (avgA < w*h) was true for all 500 images: the 7x7 no-alpha layout, terms(7,7), nx=7 and the `if hasAlpha` false branch were never fuzzed. Force full opacity on alternating iterations, which splits the run 250/250 with the seed and iteration count unchanged. All 500 still match the reference port. tiny.png is 1x1, so it has no non-zero AC content: 12 of its 37 AC coefficients sit exactly on the round(15*f) = .5 tie and 24 are within 1e-12. It passed only because a single pixel admits no summation reassociation. Give it the same header-only carve-out solid.png already had, in both test files. The four well-conditioned fixtures keep strict full byte equality. Also document the divergence class on Encode itself rather than only in test comments, add a sub-image regression spec, and use the max builtin over math.Max. The toNRGBA Rect.Min gate turns out to protect nothing — SubImage re-slices Pix so Pix[0] is the Rect.Min pixel and the loops read it correctly either way — so its comment, which claimed the opposite, is corrected. The gate is kept for now; removing it is a separate call.