The offline guarantee rests on hydration being genuinely network-free, not just
on the call sites picking the right entry point. Assert it directly: a hydrated
service must not react to a connection row appearing afterwards — which is what
the boot-time legacy migration does — and `start()` must then pick that row up,
proving the watch belongs to the live side.
Device verification of the airplane-mode cold start is not included: it needs
physical access to re-enable the test box's network. This test covers the same
invariant deterministically.
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