Adds generic child link support to ASTNode (addChild/setChild for multi/single-
valued roles, getChildren/getChild/allChildren for traversal). Function and
Variable as first child concepts of Module. Test builds Module->Function tree,
walks parent/child pointers in both directions, verifies single-valued replace.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
