E6: Add IsSystemAccount property to Account, mark $SYS account as system,
add IsSystemSubject/IsSubscriptionAllowed/GetSubListForSubject helpers to
route $SYS.> subjects to the system account's SubList and block non-system
accounts from subscribing.
E7: Add ConfigReloader.ReloadAsync and ApplyDiff for structured async reload,
add ConfigReloadResult/ConfigApplyResult types. SIGHUP handler already wired
via PosixSignalRegistration in HandleSignals.
E8: Add PropagateAuthChanges to re-evaluate connected clients after auth
config reload, disconnecting clients whose credentials no longer pass
authentication with -ERR 'Authorization Violation'.
E4: AccountImportExport with DFS cycle detection for service imports,
RemoveServiceImport/RemoveStreamImport, and ValidateImport authorization.
E5: AccountLimits record with MaxStorage/MaxConsumers/MaxAckPending,
TryReserveConsumer/ReleaseConsumer, TrackStorageDelta on Account.
20 new tests, all passing.
