Adds CVTuning to CVState, populates defaults from existing file-scope
constants in cv_init, and introduces config_load_tuning/config_save_tuning
backed by the doorcounter NVS namespace. No runtime behavior change yet;
CV code still reads the existing constants (Task 2 will migrate reads).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
