GatewayGalaxyDataWriter now accepts an optional subscribedHandleSource
delegate; TryResolveCachedOrBorrowed checks _itemHandles first then the
source, so the first write to an already-subscribed tag skips the
AddItem round-trip. Borrowed handles are not cached (subscription
registry owns lifecycle). AddItemCallCount seam confirms gateway calls.
Add IGalaxyDataWriter.InvalidateHandleCaches() and call it in
GalaxyDriver.ReopenAsync after RecreateAsync succeeds. Prior to this
fix, GatewayGalaxyDataWriter's _itemHandles and _supervisedHandles
dictionaries survived across reconnects, causing the next write to
skip AddItem and AdviseSupervisory against already-dead handles.