feat: symbolic math integration (SymPy)

[rororowyourboat]

Problem

GDS is purely structural and numerical. There is no symbolic math layer for deriving dynamics, verifying conservation laws, or proving structural properties symbolically.

Proposed Solution

Integrate SymPy as an optional dependency for:

• Symbolic derivation of state dynamics from specifications
• Conservation law verification (e.g. Hamiltonian conservation, energy invariants)
• Automatic Jacobian / sensitivity computation
• Symbolic-to-numeric compilation via sp.lambdify for simulation backends
• Proof artifacts that complement structural verification

Motivation

The Homicidal Chauffeur notebook uses SymPy to derive the full game solution: Hamiltonian, optimal controls, costate equations, and conservation proofs. This symbolic layer would complement GDS's structural verification with mathematical verification.

Scope

Optional dependency (gds-framework[symbolic] or standalone). Should not affect core compilation or verification paths.