Update exec_stmt to create globals at the top level. The var handler:

        if (streq(word, "var") or streq(word, "const")) {
            const vname: []const u8 = read_name();
            if (cur() == ':') { pos += 1; skip(); _ = read_name(); }
            if (cur() == '=') { pos += 1; skip(); }
            const val: i64 = expression();
            if (cur() == ';') { pos += 1; skip(); }
            if (!in_function) {
                setGlobal(vname, val);
            } else {
                setVar(vname, val);
            }
            return val;
        }

And set in_function = true in call_fn, restore it after:

fn call_fn(name: []const u8, args: *const [8]i64, ac: usize) i64 {
    _ = ac;
    // ... find function ...
    // Save caller's LOCAL state only
    const d: usize = save_depth;
    save_nv[d] = num_vars;
    const saved_pos: usize = pos;
    const was_in_function: bool = in_function;
    for (0..num_vars) |i| {
        save_vn[d][i] = var_names[i];
        save_vv[d][i] = var_values[i];
    }
    save_depth += 1;
    num_vars = 0;
    in_function = true;
    // Bind parameters as locals
    for (0..fn_pcounts[fi]) |i| {
        setVar(fn_params[fi][i], args[i]);
    }
    // Execute
    pos = fn_starts[fi];
    return_flag = false;
    const result: i64 = exec_block();
    // Restore locals only -- globals untouched
    save_depth -= 1;
    pos = saved_pos;
    num_vars = save_nv[d];
    in_function = was_in_function;
    for (0..num_vars) |i| {
        var_names[i] = save_vn[d][i];
        var_values[i] = save_vv[d][i];
    }
    return_flag = false;
    return result;
}

The key line: globals are never saved or restored. They survive function calls. Locals are saved on entry and restored on exit, exactly as before.