Function default_three_pane_tree 

pub(crate) fn default_three_pane_tree(
    agent_block_id: &str,
    swarm_block_id: &str,
    sysinfo_block_id: &str,
) -> (LayoutNode, String, Vec<LeafOrderEntry>)

Pure builder for the default 3-pane tree (agent | [sysinfo / swarm]). Returns (rootnode, focused_node_id, leaforder). Shared by the pre-bootstrap store-direct writer above (first launch — reducer not hydrated yet) and the post-bootstrap reducer-routed CreateWindow seed (SPEC_864 Phase 3, seed_layout_via_reducer), so the layout shape can never drift between the two paths.

Parameter order is semantic (agent/swarm/sysinfo), NOT top-to-bottom display order — sysinfo renders above swarm in the right column. Callers that build their block list positionally must keep passing them by meaning, not by visual position.