Cursors
Simple explanation
A cursor is a finger you move through the tree: down to the first or last child, sideways to siblings, up to the parent. Unlike recursive walks, it uses constant extra memory and can pause, snapshot, and resume.
Technical explanation
tree/cursor.zig keeps the current node plus a stack of (parent index, child position) entries. Moves rewrite stack entries in place, so ordinary traversal never allocates (the stack only grows on first descent into unusual depth). The cursor also tracks field context (currentFieldName/currentFieldId) and supports gotoDescendant dives, reset reuse, and copy snapshots. Cursor movement never mutates the tree.