Function let_assert

scripts/test_stringzilla.cpp:2777–2817

        sz::string words_str = "foo bar baz";
        let_assert(auto w = words_str.utf8_split().template to<std::vector<std::string>>(),
                   w.size() == 3 && w[2] == "baz");
    }

    // Test Unicode case folding
    {
        auto case_fold = [](sz::string s) -> sz::string {
            assert(s.try_utf8_case_fold());
            return s;
        };

        // ASCII uppercase to lowercase
        assert(case_fold("HELLO WORLD") == "hello world");
        assert(case_fold("ABC") == "abc");
        assert(case_fold("abc") == "abc"); // already lowercase
        assert(case_fold("123") == "123"); // no change for digits
        assert(case_fold("") == "");       // empty string

        // German Eszett - one-to-many expansion
        assert(case_fold("\xC3\x9F") == "ss");    // U+00DF -> ss
        assert(case_fold("STRAẞE") == "strasse"); // Capital U+1E9E -> ss

        // Cyrillic uppercase to lowercase
        assert(case_fold("ПРИВЕТ") == "привет");

        // Greek uppercase to lowercase
        assert(case_fold("ΑΒΓΔ") == "αβγδ");

        // Latin Extended characters
        assert(case_fold("ÀÁÂ") == "àáâ");

        // Armenian
        assert(case_fold("Ա") == "ա"); // U+0531 -> U+0561

        // Mixed case preservation for non-alphabetic
        assert(case_fold("Hello 123 World!") == "hello 123 world!");

        // Unicode characters without case folding should pass through unchanged
        assert(case_fold("日本語") == "日本語"); // Japanese (no case)
        assert(case_fold("中文") == "中文");     // Chinese (no case)
    }
}

/**