Known unreachable code in a function after a return should throw an error on --strict

e.g.:
```python
def func():
do_stuff()
return x
do_more_stuff()
```

Start by locating Coconut's handling of function bodies and the --strict validation path; the issue provides a minimal example with a statement after return. Add coverage for that example and verify that strict mode rejects the unreachable code while non-strict mode preserves its existing behavior.

Written by the indexing model from the issue text.