_downscale_to used mode="area" — a 2x2 box for the exact-2x ratio both
callers hit. A box loses on both axes at once: its passband droops well
before Nyquist (detail comes back soft) and its stopband leaks (the detail
the upsampler just invented folds back as aliasing). Soft AND speckled is
precisely the reported "sharpen leaves smearing and artifacts".
Antialiased bicubic instead. On a full-band chirp, RMS error against an
ideal brick-wall decimation drops from 0.316 to 0.244 (~23% closer).
Constant input still round-trips exactly, so the DC behaviour both callers
rely on is unchanged.
This is shared with segmented detailing's 'repair' paste, which downscales
its refined crop through the same helper — that path gets the same change.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
