Add ecommerce carton fixture tests

[Packrift]

Summary

Adds a small PHPUnit fixture using five static ecommerce carton sizes from Packrift's public packaging optimization corpus. The test covers three ordinary ecommerce order shapes and one oversized item that should not fit the sample carton set.

This is intended as a real-world input-data fixture for BoxPacker's existing ecommerce use case. It does not claim the Packrift corpus contains known optimal packing solutions; it just gives the test suite a source-backed set of carton dimensions and product URLs to exercise the packer against.

Notes

• Dimensions are converted from catalog inches to rounded integer millimetres.
• Carton dimensions are treated as static fixture dimensions; live inventory, price, freight, wall thickness, and buying approval remain outside the test.
• Source sample: https://github.com/Packrift/packaging-optimization-benchmark-corpus/blob/main/examples/ortools-carton-selection/sample_cartons.csv

Testing

• Not run locally: this machine does not currently have php or composer on PATH.
• git diff --check passes.