t27 basics 20: test, invariant and bench

The three kinds of checking block in t27 and which ones the site test runner evaluates. Lesson 20 of the t27 basics course.

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Where the numbers come from

The lesson spec public/t27/files/specs/basics/20_invariants.t27, compiled clean on 7 backends by public/t27/t27_compiler.wasm, whose tests pass in the site test runner
Compiler verdicts, error lines, code and counts quoted on this page are what the same wasm answered in node 22 when the course was authored; words without a run behind them are said to be read, not run