Function remove_stack_member 

pub fn remove_stack_member(tree: &mut LayoutNode, block_id: &str) -> bool

Remove block_id from the stack of the leaf holding it, WITHOUT removing the leaf, when that leaf has other members. If it was the visible tab, the right-hand neighbour becomes visible (the new last member when it was rightmost) — the same rule as the frontend’s removeMemberFromStack (frontend/layout/lib/stackMembers.ts). Returns false and changes nothing when block_id is in no leaf or is its leaf’s only member (removing it means removing the leaf — the caller’s job). SPEC_PANE_TABS_REDUCER_COMMANDS_2026_09_18.md §3.4.