[phys] add shape constraints, add attract-particle behavior, add spring ctor fn
[phys] remove delta & iter from VerletPhysics (iter now required arg to timestep), add clear-behaviors/constraints, bugfix remove-behavior/constraint, update ctors
[phys] bugfix add-behavior/constraint, bugfix PPhysics protocol (add delta args)
[types] optimize ptf/sweep-profile, minor other updates
[types] add PProximity impl for Line3