Scripted waveform-viewer benchmarks: assert each add-signal step worked

When you drive waveform viewers (GTKWave via Tcl, or a web viewer via page script) through seeded add-signal and jump steps, check after every add that the signal really appeared. Neither tool raises an error for a wrong hierarchy path. In one run, every add pointed at a SystemVerilog interface instance (u_tile.cpu) instead of the core instance (u_tile.u_cpu): GTKWave reported n=0 and the web viewer added nothing, so the timings measured an unchanged view. Assert n==1 per GTKWave add, and check the final row count in the web viewer. Mark a take failed and rerun it if either check fails.