Step 3: implement shunt-C with SI unit normalization
- Add model.tau_d (seconds per τ_d unit) to simulateTimeDomain; compute
dt_SI = dt * tau_d_SI so C and L shunts use correct SI time steps
- Fix accumulateShunts and updateShuntStates to use dt_SI for C/L
- Add sim/tline-shunt-c.sp SPICE netlist (2pF shunt cap at midpoint, tmax=2ps)
- Add SPICE comparison test: 42/42 passing, TOL=2e-2
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>