Both were only reachable via the periodic background sync loop removed
in 4d2cbe4 -- nothing in production calls them anymore, only tests did.
FullSync already calls backfillCore/incrementalSyncCore directly.
Rewrite the affected tests to call the *Core functions (still exported
within the package) instead, dropping the now-redundant standalone
SyncRun-recording assertion covered by TestFullSync_RecordsOneCombinedSyncRun.
Lays the groundwork for per-user accounts: CreateUser/GetUserBySub/
ListUsers plus ProvisionUser, which seeds a brand-new user's profile,
default workout-kind taxonomy, and sync state in one transaction. Depends
on later tasks scoping GetProfile/ListWorkoutKinds/GetSyncState to compile
and pass -- expected or committing to a shared branch.
