Replace SciPy ODE solvers with internal integrator and tests

[jaesparza]

Summary

• refactor the Lanchester linear solver to use its analytical form and add targeted regression coverage
• add a lightweight Runge–Kutta based solve_ivp fallback and use it in the square-law solver with dedicated tests
• update the Salvo ODE solver and packaging to avoid the SciPy dependency while comparing against the discrete simulation

Testing

• pytest

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https://chatgpt.com/codex/tasks/task_e_68d4506971e08333af9a930e380adaea