Capstone: Build a Mini validates DSL

The payoff: recreate Rails validations from scratch

Everything in this tier converges into one capstone. You'll build a working validates — a class-level macro that records rules, plus an instance valid? that runs them. When you're done, validates :name, presence: true will work on a plain Ruby class you built, and you'll understand every line of it. That's the module's thesis made concrete: Rails is Ruby you already know.

In an interview, say: "I can explain Rails validations end to end: validates is a class method that stores rules in class-level state, and valid? is an instance method that reads those rules and checks each attribute. It's the object model plus a little metaprogramming — no magic."