Subtree Runtime
src/tree/subtree.zig: Subtree is a 48-byte-ish plain record; SubtreePool owns nodes and child_indices arrays. Children are (start, count) windows of u32 indices — never pointers — so pools relocate freely and clones rewrite only integers. pushNode/pushChildren are thin ArrayList appends; cloneSubtree (in the parser) pushes the parent record first, then appends children directly into child_indices with backpatched (start, count), avoiding any temporary allocation on the hot path.