feat(logo): add stamp_logo_png() to brand file-based PNG outputs

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).

NAMESPACE

@@ -42,6 +42,7 @@ export(safe_ratio)
 export(scale_color_civilytics)
 export(scale_fill_civilytics)
 export(simpleCap)
+export(stamp_logo_png)
 export(star_subs)
 export(theme_civilytics)
 export(theme_civilytics_dark)
@@ -61,8 +62,12 @@ importFrom(ggplot2,annotation_custom)
 importFrom(ggplot2,ggplot)
 importFrom(ggplot2,theme)
 importFrom(ggplot2,theme_void)
+importFrom(grDevices,dev.off)
+importFrom(grDevices,png)
 importFrom(graphics,rasterImage)
 importFrom(grid,grid.draw)
+importFrom(grid,grid.newpage)
+importFrom(grid,grid.raster)
 importFrom(grid,rasterGrob)
 importFrom(gridExtra,arrangeGrob)
 importFrom(jpeg,readJPEG)