Function handle_muxspect_layout 

pub async fn handle_muxspect_layout(
    __arg0: State<AppState>,
    __arg1: Query<MuxspectLayoutQuery>,
) -> impl IntoResponse

GET /api/v1/muxspect/layout[?tab_id=X] — the persisted layout tree for every tab (or one), plus the layout doctor’s verdict on each.

Why this exists: layout state was previously only inspectable by reading db_layout out of SQLite by hand and walking the JSON, which is what diagnosing the cross-split minimize bugs (#2848, #2850, #2855) actually required. This applies muxspect’s existing “what is this instance doing right now” contract to layout.

It reports the PERSISTED tree, a genuinely different question from what the frontend currently renders: validate_layout_invariants normally runs at reducer write-time, so running it on demand here also catches corruption already sitting on disk, including trees written by older versions that never ran the check.

Read-only, like every route in this module except dock clear.