Add Packrift ecommerce carton fixture

[Packrift]

Summary

• add a real-world Packrift ecommerce carton fixture to the packer tests
• keep the existing bom-d-van fixture but assert packing invariants instead of exact equal-volume item ordering
• add helper assertions for packed item names and out-of-bounds placements

Validation

• PATH=/Users/farhan/Downloads/packrift-backlinks/execution/toolchains/go1.25.10/bin:$PATH GO111MODULE=off go test -timeout 20s
• GOPATH-mode go test -timeout 20s ./...
• git diff --check

The Packrift fixture source is public and dimensions/weights are kept inline in the test: https://packrift.github.io/packaging-optimization-benchmark-corpus/cartonization-solver-fixtures.html