Introduce the e2e coverage denominator: 282 behavior cells across the six
tracking modules (LLMs, MCPs, Management/UI, Reliability & Performance,
Logging & Guardrails, Other), one validated YAML row each, plus a collector
that diffs the registry against @pytest.mark.covers markers and reports
coverage per module.
The registry rows validate against a pydantic discriminated union so a row
cannot carry a field from another module. The collector is static: a
collect-only pass reads the markers, so it runs no test and needs no live
proxy. Register the covers marker suite-wide so that pass works under
--strict-markers.
This is a draft for review. Tiers are proposed rather than signed off, and a
few cells still need a support check or a prune.
