Jacobi equal-mass: μ_{12,3}/m (m_i=m) = 2/3

Sympy-exact witness of the identical-mass Jacobi 3-body reduced-mass ratio. Setup: μ_{12,3} = (m_1+m_2)·m_3/(m_1+m_2+m_3) as a three-symbol rational function. Identity: substituting m_1=m_2=m_3=m gives μ_{12,3} = 2m·m/(3m) = 2m/3, so…