> one horizontal line at a time, with one byte for Blue,Green,Red,Alpha(?) per pixel (seems like 24-bit true color).

You can (and should if you are writing software not only for yourself) ask the kernel about the framebuffer layout using FBIOGET_FSCREENINFO and FBIOGET_VSCREENINFO ioctls.

https://www.kernel.org/doc/Documentation/fb/api.txt