Credence.Semantic.MissingUseExUnitCase (credence v0.6.0)

Fixes test modules that are missing use ExUnit.Case.

ExUnit's test/2, describe/2, setup/1, etc. are macros provided by use ExUnit.Case. Without it, the module fails to compile:

error: undefined function describe/2 (there is no such import)

LLMs sometimes omit the use line because Python's unittest and pytest don't require an equivalent setup.

Auto-fix

Inserts use ExUnit.Case at the top of the module body, after any existing @moduledoc, use, import, require, or alias directives.