The package landed here with one store spec; the rest of its tests were written
three layers up, so this layer and the two above it failed the per-file
coverage gate on 191 locations while every test passed.
Adds the node half's invariant companion, the two components, the registration
apply, and the stores' failure paths. `currentValue` drops a `?? ''` that an
empty roster already returned before reaching — the same shape the later layer
arrived at, so the two converge instead of conflicting.
Yichen Jiang
c96b480bc4