Should add accuracy benchmark for expected precision w/ frame graphs

Drake was updated to use libsdformat9 in https://github.com/RobotLocomotion/drake/pull/12505, and the numerical precision in one of drake's tests had to be relaxed after this change (see comment in [multibody/inverse_kinematics/test/position_constraint_test.cc](https://github.com/RobotLocomotion/drake/blob/v0.18.0/multibody/inverse_kinematics/test/position_constraint_test.cc#L87-L93)). We should add a benchmark related to the numerical accuracy of pose calculations involving frame graphs, which may help illustrate the underlying issue.

Start by reading the referenced Drake position constraint test and its comments about relaxed numerical precision. Then trace the corresponding frame-graph pose calculations in sdformat and identify where a benchmark can exercise them. Done means the repository contains a repeatable accuracy benchmark that demonstrates the expected precision and passes with the project’s test suite.

Written by the indexing model from the issue text.