DOC: add TR-037 on B⁰ → D*⁺π⁻π⁰ spin-1 model

[redeboer]

Tip

Preview of TR-037 here.

✨ New features

• Add TR-037, which models $\overline{B}^0\to D^{*+}\pi^-\pi^0$ twice: as a three-body decay with a stable spin-1 $D^{*+}$ in AmpForm-DPD, and as the four-body decay $\overline{B}^0\to D^0\pi^+\pi^-\pi^0$ in AmpForm with the $D^0\pi^+$ subsystem pinned to the $D^{*+}$ resonance. The report derives the fifteen helicity couplings required for a spin-1 daughter, checks helicity-sum interference cancellations, and compares one-dimensional mass projections and angular distributions between the two formulations.

Squash commit messages

* DOC: add TR-036-style mass projections and Dalitz plot to TR-037
* DOC: clarify TR-037 and improve decay diagram rendering