QArena verifier bundle: decoder (QEC decoder accuracy)

Requirements: Python 3.10 or newer, standard library only.

Ranked instances keep their labels (the true observable flips) private on the QArena server. This bundle
cannot score a ranked file: run_one.py answers 'hidden key unavailable' for it. Check a ranked file's format:
  python3 -I verifiers/decoder_format.py <instance> my_file.json

Development instances (names ending in -dev) include their public labels and are scored exactly:
  python3 -I verifiers/run_one.py decoder surface9-si1000-v1-dev example/surface9-si1000-v1-dev.json

Detector data, circuits and error models: https://qarena.ai/b/decoder/files
Verifier version ff9e154722c5. Source: https://qarena.ai/verifiers/
