Function agg_plan

crates/kernel/src/shard.rs:359–364

Lower the bisection tree to an arity-bounded fold plan. The binary tree is collapsed* so each agg call folds up to `arity` whole subtrees (never splitting a subtree across calls), keeping the agg-proof count ~`N/(arity-1)` — a strict binary fold would be ~`N` agg proofs, dominating cost. Children of each agg are emitted in shard order so the subject merkle-fold matches the guest's left-associative

(tree: &AggNode, arity: usize)

/// leaf proof; `Agg(children)` folds those slots' proofs in one agg call.
#[derive(Clone, Debug, PartialEq, Eq)]
pub enum FoldOp {
  Leaf(u32),
  Agg(Vec<usize>),
}

/// Lower the bisection tree to an arity-bounded fold plan. The binary tree is
/// *collapsed* so each agg call folds up to `arity` whole subtrees (never
/// splitting a subtree across calls), keeping the agg-proof count ~`N/(arity-1)`
/// — a strict binary fold would be ~`N` agg proofs, dominating cost. Children of