Update check to reset globals:

fn check(input: []const u8, expected: i64) void {
    source = input;
    pos = 0;
    num_vars = 0;
    num_globals = 0;   // ← add this
    fn_count = 0;
    return_flag = false;
    save_depth = 0;
    in_function = false;  // ← add this
    skip();
    const got: i64 = program();
    // ...
}

Now test:

    check("var x: i64 = 42; fn get() i64 { return x; } get()", 42);

ok 42. The function sees the global.