Add tools/gen_skin_template.py to generate a labeled 642x420 PNG template
for creating custom spritesheets. Move rg35xx device scripts from tools/
to device/rg35xx/. Point prev_sprite at its own cell (bottom-center) so
Prev and Next can have distinct icons.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
