feat: SubscribeNodeGroupStatus RPC for per-node-group chain status

[l0gun0v]

Design: see the design doc (Notion) — problem statement, the group key, the full stream contract and the phase-2 plan live there.

Adds a dedicated RPC for per-node-group chain status:

rpc SubscribeNodeGroupStatus (SubscribeNodeGroupStatusRequest) returns (stream SubscribeChainStatusResponse) {}

message SubscribeNodeGroupStatusRequest {
}

message ChainDescription {
  ChainRef chain = 1;                   // unchanged
  repeated ChainEvent chain_event = 2;  // unchanged
  string node_group_id = 3;             // set on node-group streams
}

• The new RPC reuses SubscribeChainStatusResponse, so consumers parse exactly the events they already know — the difference is that each ChainDescription describes one node group instead of the merged per-network view.
• node_group_id is an opaque "<client_type>:<methods_hash8>" string (e.g. erigon:3fa9c21b); clients must not parse it. It is always empty on SubscribeChainStatus.
• A group's ChainStatus going AVAIL_UNAVAILABLE is its removal signal.
• SubscribeChainStatus is untouched: existing consumers see no change, the two views never mix in one stream, and rollback is "stop calling the new RPC".

Consumers: drpcorg/nodecore#316 (server), drpcorg/dproxy#2804 (aggregator) — both pin their submodule to this branch and need this merged first.