Composable ExUnit helpers for Rust generated by defrust and defnif.

The helpers return source or boolean values so tests keep using ExUnit's ordinary assert macro, pattern matching, and diffs:

defmodule MyApp.NativeTest do
  use RustQ.Test, async: true

  test "generates and exports the native boundary" do
    assert rust_source!(MyApp.Native, :sum_impl) =~ "fn sum_impl"
    assert nif_exported?(MyApp.Native, :sum, 1)
    assert RustQ.valid?(rust_source!(MyApp.Native), "my_app_native.rs")
  end
end

rust_source!/1 returns a module's complete generated Rust. rust_source!/2 renders one named generated function, which keeps source assertions focused and prevents matches across unrelated functions.