FEAT: add TR-036 on D⁺ → π⁺π⁺π⁻ Dalitz model - #44

✨ New features

• TR-036 on $D^+ \to \pi^+\pi^+\pi^-$ with Dalitz-plot decomposition. The report builds a Bose-symmetric, nine-resonance isobar model with QRules, formulates the symbolic helicity amplitudes with AmpForm-DPD, and evaluates the Dalitz density with TensorWaves. It renders the invariant, angular, and dynamics definitions from the model expressions themselves, verifies Bose symmetry of the complex amplitude (not only of the intensity), and plots the coherent density on a $1000 \times 1000$ grid over the physical domain.
• The resonance content combines the scalar isobars of the CLEO analysis with the additional $\omega(782)$, $\rho(1450)$, and $\rho(1700)$ vectors of the LHCb analysis. Coupling magnitudes are fixed by matching ratios of diagonal component integrals to the published fit fractions, which makes them independent of the overall amplitude normalization and of each lineshape's own normalization. Phases are adopted as illustrative starting values only.
• The notebook works around a form-factor kinematics issue in AmpForm-DPD v0.2.4, where the decay vertex receives $s^2$ instead of $s$ and the production vertex receives the pole mass $m_R$ instead of $\sqrt{s}$ as a daughter mass. The substitution is applied to the model amplitudes and asserted afterwards.
• Two new bibliography entries back the resonance content: the CLEO Dalitz plot analysis (arXiv:0704.3954) and the LHCb amplitude analysis with the $\pi^-\pi^+$ $S$-wave measurement (arXiv:2208.03300).
• The report illustrates the reaction's decay chains as Mermaid diagrams, rendered from the qrules.io.asmermaid output.
• One-dimensional mass projections of the coherent density onto each pair-mass-squared variable, integrated on the Dalitz grid and normalized to unit area. The report cross-checks that the $s$ and $t$ projections coincide as required by Bose symmetry and that each projection integrates back to the coherent intensity.

🖱️ Developer experience

• Agent configuration files (.agents/, .claude/, .codex/, AGENTS.md, CLAUDE.md) are ignored, so local agent instructions stay out of the repository.
• The Sphinx build gains sphinxcontrib-mermaid support, with a stylesheet that renders diagrams at natural size and keeps edge labels legible, so future reports can embed Mermaid diagrams as well.
• The docs theme's fullscreen button is disabled.

Squash commit messages

* DOC: add CLEO and LHCb three-pion Dalitz references to bibliography
* DOC: add TR-036 mass projections of the Dalitz density
* DOC: illustrate TR-036 decay chains with Mermaid
* DX: disable fullscreen button in the docs theme
* DX: ignore agent configuration files