New self-contained Metrics explainer (metrics_page.py) served at
/metrics and embedded in a Metrics tab: validity gate, the three axes,
editing renormalization, with formulas and the interface mating-group
illustration (vendored under assets/metrics/ via LFS, served by the
/metrics-assets route). Submit's merge path passes the submission name
and the /metrics base URL to the report generator so hosted reports
title themselves and their metric pills deep-link to the explainer.
Co-authored-by: Cursor <cursoragent@cursor.com>
