Regression from deleting the provider. sync() gated on
providerResolver.resolve() != null, and OWN resolves to no provider by
design — so from that commit no due reminder was ever armed in what had
just become the default mode, and clearAll() cancelled any that survived
the upgrade.
The gate is now ProviderResolver.canReadStore(): OWN is always readable,
and only EXTERNAL can fail, for the two reasons it ever could. Putting
the decision on the resolver rather than inside the scheduler is what
makes it testable at all — ReminderScheduler needs Context and
AlarmManager, which is why nothing caught this.
Also brings ARCHITECTURE.md and ROADMAP.md in line with the branch: one
module, OWN/EXTERNAL, the four Room tables, expansion at read time, the
import and startup gate, and the manifest surface that no longer declares
a provider or any permission of its own.