Method capacity

include/libfork/core/impl/stack.hpp:97–100

* @brief Capacity of the current stacklet's stack. */

     * @brief Capacity of the current stacklet's stack.
     */
    [[nodiscard]] auto capacity() const noexcept -> std::size_t {
      LF_ASSERT(m_hi - m_lo >= 0);
      return static_cast<std::size_t>(m_hi - m_lo);
    }
    /**
     * @brief Unused space on the current stacklet's stack.
     */