civilytics_logo()/make_logo_grob() brand ggplot/grobs, but flextables and
other outputs are rendered to PNG first and can't use them. stamp_logo_png()
is the raster analogue: it resolves the SAME brand asset that make_logo_grob()
uses (via the type/variant switch) and composites it into a corner of an
existing PNG in place, so file-based tables stay visually consistent with
logo-branded plots.
Dependency-free by design — uses only png + grid + grDevices (all already
imported), no magick. Configurable type/variant/position/width/margin;
preserves the image's pixel dimensions. Adds tests (21 assertions).
