Allow FIFO inputs

[toddstrader]

vcddiff currently only works on seekable files. That means while this obviously has no diffs:

vcddiff foo.vcd foo.vcd

this (incorrectly) does:

vcddiff <(cat foo.vcd) <(cat foo.vcd)

and more importantly so does:

vcddiff <(fst2vcd foo.fst) <(fst2vcd foo.fst)

This is not ideal for very large VCDs. We could instead make temporary files and open them instead but that's less ideal for cases which do reasonably fit in memory. In any case, this is less surprising than getting a diff between a file and itself.

[wsnyder]

Thanks. Using a personal Verilator branch, edit ci/ci-install.bash to point to your branch with this change, and make sure all tests pass on all platforms. I'm especially worried about Windows.

[toddstrader]

I'm with you that adding more system calls may run into complications on Windows. However, I don't think ci-install.bash is used for Verilator Windows CI. And it appears that we do very little testing in ci-win-test.ps1, mainly we just run examples/cmake_tracing_c:
https://github.com/toddstrader/verilator/actions/runs/19572878666/job/56050498001

I'd suggest getting test_regress going on Windows in the Verilator CI, but I'm guessing there are reasons that's not already happening. So would a better way forward here be to spin up some simple, multi-OS CI for this repo? This PR is clearly not urgent, but if so I can take a stab at that when time allows.

[wsnyder]

Yes, a pull to add here all of the same runner flavors as we use on Verilator would be appreciated. It was just that no one got to it.

[wsnyder]

Thanks for the CI tests, please merge master into this, add a test, and once passes will be good to merge.

[toddstrader]

As predicted, this doesn't play nice with Windows. And based on my limited research, there's not a direct analogue to unix fifos in Windows. So I'm just leaving this as a non-Windows thing for now.