v2.1.0

Fix: fuel_cost and grid_cost columns in the trajectory DataFrame no longer overcount by one segment. fuel_cost is now derived directly from the mass trajectory (-diff(mass)), matching the physical fuel burn exactly.

Breaking: interpolant_from_dataframe now defaults to shape="bspline" (was "linear"), aligning with cached_interpolant_from_dataframe and the opentop gengrid CLI. Linear interpolants cause IPOPT to oscillate chaotically on non-trivial grids.

Full Changelog: v2.0.2...v2.1.0