[Packrift]

Summary

• add a compact Packrift ecommerce carton fixture to the 3D solver regression suite
• exercise multi-carton packing with real carton dimensions, item quantities, and an inventory cap
• verify the fixture packs all 8 items, uses only declared carton types, respects the capped 24x10x8 carton, and produces a finite-cost solution

Fixture source: https://packrift.github.io/packaging-optimization-benchmark-corpus/cartonization-solver-fixtures.html

Validation

• cargo fmt --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• cargo doc --manifest-path crates/bin-packing/Cargo.toml --no-deps --document-private-items