FEAT: implement final-state polarization - #200
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Add a polarized_final_states argument to DalitzPlotDecompositionBuilder.formulate() that attaches one additional Wigner-D function per amplitude for each selected final-state particle, connecting its helicity (summed coherently) to the helicity of its decay product (summed incoherently), weighted by final-state decay couplings H^fs. With the default couplings of 1, the intensity is identical to the model without the additional Wigner-D functions; for weak decays such as Sigma+ -> p pi0, the couplings follow from the decay-asymmetry parameter via the new formulate_weak_decay_couplings() helper. This makes production-coupling phases visible that are flat directions in a Dalitz-plot-only fit. The new polarization example notebook demonstrates this on J/psi -> K0 Sigma+ pbar: the proton direction in the Sigma+ rest frame becomes sensitive to the Sigma+ polarization, while the Dalitz distribution is unchanged.
The decay angles (phi_k, theta_k) of a polarized final-state particle cannot be computed from the Mandelstam variables, since the decay product lies outside the three-body decay plane. Formulate them with AmpForm's compute_helicity_angles() over an extended four-body topology in which final-state particle k decays further, and include the resulting boost-chain expressions in AmplitudeModel.variables. The expressions take four-momentum arrays p1, p2, p3 and q<k> (the analyzed decay product) in the center-of-momentum frame as input, following the boost chain of the reference subsystem.
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