Prepare fixture3-ddmin for public release

[tartakovsky]

Summary

• make fixture3-ddmin publishable as a standalone crate
• add crate README, rustdoc, examples, release metadata, and docs.rs metadata
• add DDMin verification, docs, and publish dry-run steps to CI

Verification

• scripts/verify-all.sh
• scripts/verify-ddmin.sh
• python3 scripts/verify-rust-workspaces.py --g3rs
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps --manifest-path packages/ddmin/Cargo.toml -p fixture3-ddmin
• cargo publish --dry-run --allow-dirty --manifest-path packages/ddmin/crates/ddmin/Cargo.toml