fix(connection): make named channels node-local

[smartinio]

Problem

Named channels are currently registered with :global, which makes them effectively cluster-global across connected BEAM nodes.

In distributed deployments, this results in:

1. One node creates the named channel registration
2. Another node trying to connect the same named channel sees :already_started, then fails to resolve the channel locally and returns {:error, :no_connection}.

Root cause

GRPC.Client.Connection registers named channels globally via :global, but the selected channel state is still read from node-local :persistent_term. I.e. registration is shared across nodes, while channel state is not.

Fix

Scope named channels to the current BEAM node instead of sharing them across connected nodes.

• Start a node-local Registry for client connections
• Register named channels through that Registry instead of :global
• Update connection tests to look up the node-local registration

Test plan

• Added regression test that verifies two connected peer nodes don't conflict on the same name
• Added a small shim for :peer to pass OTP24 interop tests, since :peer is an OTP25+ module