Function group_by_stack 

pub fn group_by_stack(stacks: &[Option<Stack>]) -> StackGrouping

Partition a slice of optional stack descriptors into groups sharing the same connected stack.

The grouping key is (trunk, sorted diff set), so worktrees in the same connected stack always collapse into one group regardless of the Stack::diffs vector order returned by current_stack. Members within each group are ordered by their branch’s position in the representative stack’s diffs list (bottom→top).

Groups are returned in deterministic order: sorted by (trunk, sorted branch set).

When all entries are None (stack model inactive or --no-stack), groups is empty and every index appears in ungrouped — callers get identical flat-list behavior.