The Foundation is the well the whole corpus rests in — and a well is just mass, bending the paths around it. Launch a body: too slow and it falls in and orbits; fast enough and it escapes forever. The line between is the escape velocity, and it grows with the mass.
A real inverse-square field: each body is integrated with a = −G·M·r̂ / r² (semi-implicit Euler), the same law that makes orbits. The escape speed at the launch radius is v_esc = √(2GM/r), computed and shown; launch below it and the body is bound (it orbits, ellipse), at or above it and the trajectory is unbound (it leaves and never returns). Bound-vs-free is decided by the sign of the specific orbital energy ½v² − GM/r, checked live.
'The Foundation is the well the corpus rests in' is the domain's own figure. Under the hood is ordinary Newtonian two-body motion — real gravity, real escape velocity. The 'corpus' is not literally mass; the well is an image for how the whole binds around a centre.