hscontrol: fix tag updates not propagating to node self view

When SetNodeTags changed a node's tags, the node's self view wasn't
updated. The bug manifested as: the first SetNodeTags call updates
the server but the client's self view doesn't update until a second
call with the same tag.

Root cause: Three issues combined to prevent self-updates:

1. SetNodeTags returned PolicyChange which doesn't set OriginNode,
   so the mapper's self-update check failed.

2. The Change.Merge function didn't preserve OriginNode, so when
   changes were batched together, OriginNode was lost.

3. generateMapResponse checked OriginNode only in buildFromChange(),
   but PolicyChange uses RequiresRuntimePeerComputation which
   bypasses that code path entirely and calls policyChangeResponse()
   instead.

The fix addresses all three:
- state.go: Set OriginNode on the returned change
- change.go: Preserve OriginNode (and TargetNode) during merge
- batcher.go: Pass isSelfUpdate to policyChangeResponse so the
  origin node gets both self info AND packet filters
- mapper.go: Add includeSelf parameter to policyChangeResponse

Fixes #2978

hscontrol/state/state.go

@@ -719,7 +719,18 @@ func (s *State) SetNodeTags(nodeID types.NodeID, tags []string) (types.NodeView,
 		return types.NodeView{}, change.Change{}, fmt.Errorf("%w: %d", ErrNodeNotInNodeStore, nodeID)
 	}
 
-	return s.persistNodeToDB(n)
+	nodeView, c, err := s.persistNodeToDB(n)
+	if err != nil {
+		return nodeView, c, err
+	}
+
+	// Set OriginNode so the mapper knows to include self info for this node.
+	// When tags change, persistNodeToDB returns PolicyChange which doesn't set OriginNode,
+	// so the mapper's self-update check fails and the node never sees its new tags.
+	// Setting OriginNode ensures the node gets a self-update with the new tags.
+	c.OriginNode = nodeID
+
+	return nodeView, c, nil
 }
 
 // SetApprovedRoutes sets the network routes that a node is approved to advertise.