Function seed_layout_via_reducer 

pub(crate) async fn seed_layout_via_reducer(
    state: &AppState,
    tab_id: &str,
    new_tree: LayoutNode,
    focused_node_id: String,
    leaforder: Vec<LeafOrderEntry>,
    magnified_node_id: String,
) -> Result<(), String>

SPEC_864 Phase 3 — single-writer layout seeding. Dispatch a LayoutSetTree (tree + focus + leaforder as client slices) and persist it inside ONE hold of the reducer mutex, so persist order equals dispatch order — the same contract as UpdateObject’s Phase-2 route in object.rs. Replaces the wcore-direct seeders (write_default_three_pane_layout post-bootstrap caller, setup_torn_off_block_layout): the reducer’s TabRecord.rootnode is authoritative, and the persist subscriber is the sole db_layout writer on this path.

On SQLite apply failure the reducer is rolled back to an empty tree (the seed target is a fresh tab whose row was empty) inside the same lock hold. Publishes events on success. Requires the tab to be known to the reducer — true for every caller (all run after reducer-routed CreateTab / sagas); the pre-bootstrap first-launch seed (ensure_initial_data → seed_default_layout) deliberately stays store-direct because the reducer isn’t hydrated yet (spec site #3).