Display tessellation plans (1/4): planner with shared edge nodes (#802)

Adds TessPlan and the kernel planner (shared edge nodes, structured and Coons grids, constrained Delaunay for trimmed faces) behind Body::display_plan. Nothing drawn changes yet.

Refs #303

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/geom/src/lib.rs

@@ -16,6 +16,7 @@ pub mod fill;
 mod mesh;
 mod plane;
 mod profile;
+pub mod tess;
 pub mod tspline;
 mod vec;