Type Alias FusedMappingValue<S>

Value form for one window in a FusedMapping. Either:

• A bare AggregateMap (key = source column, value = reducer)
• A bare AggregateOutputMap (key = output alias, value = { from, using })
• An FusedMappingElaborated wrapper for per-window options

The two bare forms match the existing live.rolling(window, mapping, ...) value shapes — fused rolling accepts either uniformly.