Interface IRemoteDispatchSelector

Selects the physical dispatch target for a region — the second flow of Layer 2, "remote dispatch selection from location in the region tree".

Given a target region node (a user's residency-derived region), this selector answers which server should send, honouring the routing precedence:

1. Residency (hard): the serving node must lie within the target's residency ceiling. Enforced by IRegionTree.selectServingNode(IRegionNode), which never falls back past the ceiling.
2. Push capacity (hard): a server over its push throughput roof is not chosen; the slice spreads to a sibling in-region or spools. Note this is the push roof, independent of the session roof used by session routing.
3. Region affinity and push load (soft): among residency-legal, non-saturated servers, the least-loaded and nearest is preferred.

The selector owns only the region-level decision (residency and reachability); the chosen region then places the send on one of its own servers by push load, which is the load balancer's competence and is not duplicated here.

On a single-server deployment this selector always returns a local, reachable target, so the distributed machinery is dormant and the caller's flow is identical to the non-distributed case.