New domain-agnostic bottom-up tidy-tree module (Reingold-Tilford contour pack)
operating on abstract { id, width, children } nodes — zero generated-API
imports. First rung of the TDD ladder: a single leaf lays out at x=0. The full
contour/centring machinery is in place; subsequent commits add tests that
exercise it.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
