FEAT: formulate serialized coordinate maps

[redeboer]

Closes #214

✨ New features

• Add formulate_kinematic_map(), which converts a serialized invariant-mass/helicity-angle coordinate into a complete symbolic mapping for sigma1, sigma2, and sigma3.
• Derive the mapping in closed form from the Kallen function without invoking sympy.solve() during evaluation.
• Return an immutable mapping and require explicit distribution selection when a workspace contains multiple distributions.
• Reject ambiguous coordinate definitions and helicity-angle orientations outside the supported cyclic convention.

formulate_kinematic_map is re-exported from ampform_dpd.io.serialization and remains independent of numerical backends.

🖱️ Developer experience

• Cover forward/inverse consistency at a serialized reference point, multi-distribution selection, and invalid coordinate metadata.

📝 Documentation

• Document that the helicity angle is measured between particle i and spectator k in the (ij) isobar rest frame for cyclic pairs (1, 2), (2, 3), and (3, 1).

Squash commit messages

* DOC: document cyclic helicity-angle convention
* DX: test kinematic map against scattering angle