Switch checking on

```text
## Switching checking on

This Sather checks contracts when the compiler is given `-chk`:

sacomp -chk mine.sa -main MAIN -o mine

Without it, `pre` and `post` are documentation the compiler has read and
agreed is well-formed, and nothing more. The tests for this exercise are
built without `-chk`, so they cannot catch a broken contract for you; they
check that the routines do the right thing when used properly.
```

This is currently in `introduction.md` - we should instead put something like this in the docs, and have the test runner (and exercism cli) always use `-chk`

Start with introduction.md, then inspect the test runner and Exercism CLI handling of compiler arguments. Move the switching-checking guidance into the appropriate documentation and ensure both runners invoke sacomp with -chk; the tests should exercise contracts with checking enabled.

Written by the indexing model from the issue text.