For plotting you use the 'plot' command. There is no 'probe' command in a .control section. If you look for example at \examples\xspice\state\state-machine.cir, plotting is done via gtkwave, using the exported vcd file. Adding a line plot n_one clk n_zero msb lsb digitop will allow plotting the digital data with ngspice internals only.
Compilation Error in options.c
spicious warning
Fixed in pre-master-47 2c0aabd76 ("Remove linker warnings (VS2022)", 2026-04-25)
spicious warning
Fixed in pre-master-47 2c0aabd76 ("Remove linker warnings (VS2022)", 2026-04-25)
ngspice crashes in eprvcd (with fix)
Fixed in git branch pre-master-47.