IO only needed Engine for the best-fit cache, which now goes through
IBestFitStore. Removing the reference stops every IO consumer pulling in
the engine and ONNX Runtime. A guard test keeps the assembly from
referencing it again; IO.Tests references Engine itself for its whole-job
round-trip tests.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
