chore(scripts): move corpus-balance shell scripts from root to scripts/

balance_corpus.sh, balance_corpus_fast.sh, balance_smtlib.sh, and
balance_smtlib_optimized.sh now live in scripts/ alongside the existing
shell/Julia maintenance scripts. Verified: no Justfile recipe, CI
workflow, or build script references these by their old root path.

https://claude.ai/code/session_01YPqu7gti4azBach6ZvpRFJ

Author: claude