Display tessellation plans (2/4): size rules and the facet angle (#803)

Per-face target side lengths, the caller's facet angle, refined grid lines, and fixes for underflowing coordinates and straight edges ending exactly on their vertex. Nothing drawn changes yet.

Refs #303

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

Cargo.toml

@@ -40,6 +40,7 @@ truck-topology = "0.6"
 # constrained Delaunay + Ruppert refinement for trimmed faces (MIT OR Apache-2.0); already in the
 # tree through truck-meshalgo.
 spade = { version = "2.15", default-features = false, features = ["std"] }
+rayon = "1.12"
 
 # packages (3MF)
 zip = { version = "3", default-features = false, features = ["deflate"] }