Linear separability and XOR

In the book: Chapter 1 (AND/OR plots); Chapter 2, §"The Limitations of the Perceptron". Builds on: The formal model.

Planned. Defines linear separability of a labeled dataset, exhibits separating hyperplanes for AND and OR, and proves that no $(\mathbf{w},b)$ separates XOR — the four-inequality contradiction argument, done honestly.