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This PR will close #411
As discussed in #411, this PR allos our end user build substrait-rs without protoc.
This PR was primarily authored with Codex using GPT-5-Codex and then hand-reviewed by me. I AM responsible for every change made in this PR. I aimed to keep it aligned with our goals, though I may have missed minor issues. Please flag anything that feels off, I'll fix it quickly.