Coverage-first safety net for the E3 S1 Plex god-module split. The movie/show
watch-state predicates are currently hand-duplicated in unified_library (the
grid) and facets.wsq (the facet aggregation); this pins the invariant the
facets docstring promises — with no metadata filter active, a facet's genre /
content-rating counts equal the same counts over the grid's visible items — for
every (scope × show) combination, deriving the expected counts from the grid's
own response. Locks the two branch-sets together before they merge into one
shared helper, so the merge is provably behaviour-equivalent.
Mutation-verified: dropping the facets hidden-exclusion branch makes both tests
fail (Drama 3≠2, R 2≠1). Green on the current code (348 DB-lane pass).
peter
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