Remove native-client delegation in calculate_margin_and_volume() so the
pdmt5 linear estimate can no longer overstate affordable volume on
foreign-denominated instruments or CFDs. The module's own
calculate_volume_by_margin() verifies each candidate volume against
order_calc_margin() and returns the largest stepped lot that fits the
margin budget.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
