contracts: add BrowseChildren RPC for lazy hierarchy browse

src/ZB.MOM.WW.MxGateway.Contracts/Protos/galaxy_repository.proto

@@ -30,6 +30,12 @@ service GalaxyRepository {
   // increasing per server start; gaps indicate the per-subscriber buffer dropped
   // older events because the client was too slow.
   rpc WatchDeployEvents(WatchDeployEventsRequest) returns (stream DeployEvent);
+
+  // Returns the direct children of a parent object (or the root objects when
+  // `parent` is unset). Designed for OPC UA-style lazy expand: clients walk
+  // one level at a time instead of paging the full hierarchy. Filters mirror
+  // DiscoverHierarchy exactly. Backed by the same shared hierarchy cache.
+  rpc BrowseChildren(BrowseChildrenRequest) returns (BrowseChildrenReply);
 }
 
 message TestConnectionRequest {}
@@ -141,3 +147,44 @@ message GalaxyAttribute {
   bool is_historized = 10;
   bool is_alarm = 11;
 }
+
+message BrowseChildrenRequest {
+  // Parent selector. Empty oneof returns root objects (parent_gobject_id == 0).
+  oneof parent {
+    int32 parent_gobject_id = 1;
+    string parent_tag_name = 2;
+    string parent_contained_path = 3;
+  }
+
+  // Maximum number of direct children to return. Server default 500; cap 5000.
+  int32 page_size = 4;
+  // Opaque token returned by a previous BrowseChildren response. Bound to the
+  // cache sequence, parent selector, and the filter set; a mismatch returns
+  // InvalidArgument.
+  string page_token = 5;
+
+  // --- Filter parity with DiscoverHierarchy. AND-combined. ---
+  repeated int32 category_ids = 6;
+  repeated string template_chain_contains = 7;
+  string tag_name_glob = 8;
+  optional bool include_attributes = 9;
+  bool alarm_bearing_only = 10;
+  bool historized_only = 11;
+}
+
+message BrowseChildrenReply {
+  // Direct children matching the filter, sorted areas-first then by
+  // case-insensitive display name (same order as the dashboard tree).
+  repeated GalaxyObject children = 1;
+  // Non-empty when another page of siblings is available.
+  string next_page_token = 2;
+  // Total matching direct children of the parent (post-filter).
+  int32 total_child_count = 3;
+  // Parallel array, indexed with `children`. True when the child has at least
+  // one matching descendant under the same filter set. Lets a UI choose
+  // whether to draw an expand triangle without an extra round trip.
+  repeated bool child_has_children = 4;
+  // Cache sequence this reply was projected from. Clients may pass it back as
+  // part of the page_token contract. Mismatch on the next page -> InvalidArgument.
+  uint64 cache_sequence = 5;
+}