Credence.Semantic.NoUnderscoreInExpression (credence v0.8.0)

Fixes compiler errors caused by using _ in expression position, such as a tuple key in a for-comprehension body.

The Elixir compiler rejects _ in expression position:

for _ <- 0..(n - 1), into: %{} do
  {_, :infinity}    # ← error: invalid use of _
end

The fix renames the _ generator to a fresh variable (idx) and updates all references in the comprehension body:

for idx <- 0..(n - 1), into: %{} do
  {idx, :infinity}
end