Simple Harmonic Motion (SHM) simulator — with steps and energy

The solver computes omega = sqrt(k/m) for the spring-mass system, reports the period T = 2pi/omega, and evaluates x(t), v(t), a(t), and the energy terms K = 0.5 m v^2 and U = 0.5 k x^2. The analytic, Euler-Cromer, and RK4 solvers all log their steps in the How it's calculated list.

Euler-Cromer is fast and keeps energy bounded for larger time steps, making it useful for exploratory lessons. RK4 is higher order and matches the analytic curve to within the required 1e-3 tolerance while still emitting the same time series table and CSV export.