Step 4: add shunt-L SPICE netlist and validation test
- sim/tline-shunt-l.sp: 10nH shunt inductor at midpoint (τ_L=400ps)
- SPICE comparison test: inductor rise-then-decay waveform vs MoC
- 43/43 tests passing, TOL=2e-2
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>