The defect was invisible from the outside because every behavioural test in
the suite seeded a tagged reading. Two checks close that:
A structural guard that reads the dashboard surfaces and fails when a tile
derives presence from a list of enum buckets with no null / unknown arm and
no written reason, following one hop through a named constant. It carries
its own positive and negative controls, so a refactor that silently stops
matching anything fails here rather than going quiet.
An integration test that seeds twenty readings with a genuinely NULL context
against a real Postgres and reads the tile back through the same resolver
the page renders with: eligible, latest value intact, and a tagged reading
still filed under its own context.
Refs #943
