FEAT: complete symbolic deserialization

[redeboer]

Closes #213

✨ New features

• Support serialized Polynomial parametrizations as sums of coefficients[n] * sigma**n and MomentumPower form factors through powers of breakup momentum.
• Accept the published generic-function notation, including 1im for the imaginary unit and invariant names such as m_12_sq.
• Formulate named functions independently of their presence in a selected distribution expression tree.
• Formulate the published Lambda-b and multi-distribution COMPASS models without compatibility builders or patched definitions.

❗ Behavioral changes

• Build multichannel Breit-Wigner propagators from analytically continued PhaseSpaceFactorComplex terms and squared FormFactor terms so expressions remain defined below threshold.
• Name couplings for all-LS chains from their LS quantum numbers while preserving existing helicity coupling names for helicity-vertex chains.
• Derive decay products and resonance nodes from serialized vertex topology instead of requiring injected helicities.

🐛 Bug fixes

• Read child spins from final-state definitions when formulating chains with integer nodes.

🖱️ Developer experience

• Cover LS formulation end to end, polynomial and momentum-power expressions, published numeric notation, and complex phase space above and below threshold.

Squash commit messages

* BEHAVIOR: derive LS coupling names from l and s
* BEHAVIOR: use complex phase space in multichannel BW
* FIX: read child spins from final-state definitions