Language basics

The part of Dart syntax you use every day: choosing between `var`/`final`/`const` and the compile-time versus runtime distinction, the three parameter forms and how closures capture variables, collections and `Iterable` laziness, string immutability and Unicode traps, plus Dart 3 records and pattern matching. The stage focuses on the traps interviews ask about most — `final` versus `const`, an `int` not being a `double`, a lazy chain executing twice, and exhaustiveness over a `sealed` hierarchy.