feat(katok): update to v0.3.1

[vkehfdl1]

Summary

• publish the privacy-history gate and synthetic fixture remediation
• ship atomic search-index recovery and semantic bulk-loading improvements
• bump the Rust crate to 0.3.1

Verification

• rustup run 1.91.0 cargo fmt --all -- --check
• rustup run 1.91.0 cargo clippy --all-targets -- -D warnings
• KATOK_EMBEDDER=local-test rustup run 1.91.0 cargo test --all-targets
• rustup run 1.91.0 cargo publish --dry-run
• python3 scripts/verify_release_config.py
• bash tests/verify_history_privacy.sh
• python3 scripts/verify_history_privacy.py . origin/main..HEAD
• cargo run -- --help
• invalid CLI invocation exits non-zero with usage output