`CreateSmartPlaylist` issued its `INSERT ... RETURNING` through
`QueryContext`, which routes to the query-only read pool, and failed
with "attempt to write a readonly database (8)". No smart playlist
could be created at all, in any real build.
It was invisible because `NewTestDB` shares one in-memory connection
and leaves `readDB` nil, so `reader()` hands back the *writer* under
test: every unit test of that path exercised a handle production does
not have. `TestNoWritesOnTheReadPool` walks the tree for the whole
class, in the same spirit as `TestNoDirectRuntimeEmits` and for the
same reason — a lint pass only sees one build configuration.
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