qec_code, qec_decode and qec_threshold call the engine's error correction
in-process, each answer sealed in a job's form, and run() takes mitigate=.
The tests decode every error of weight up to two on the distance-5 colour
code to a trivial residual, check a certificate and a forged one, and hold
each answer to the command line's.
Co-Authored-By: Claude Opus 5.5 <[email protected]>
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