Here's the shader. Two changes, actually.

First I used a Round function instead of Gain. This makes anything < 0.5 equal to 0, and anything >= 0.5 equal to 1. So the Blender node is literally switching between textures instead of blending them. I thought Gain would do this quite well, but no.

I also added a Math Add node immediately after the Diffuse to adjust the shadow detection threshold, but the render above didn't need any adjustment so the value is zero. You might need this for different levels of lighting in your scenes.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)