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.
Dedicated adversarial coverage for the core security property: no store
method can read or mutate another user's profile, workout kinds/paces, or
sync state/runs, even when handed that other user's real row id.
