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>

crates/kernel/Cargo.toml

@@ -20,3 +20,6 @@ truck-meshalgo = { workspace = true }
 truck-stepio = { workspace = true }
 truck-topology = { workspace = true }
 spade = { workspace = true }
+
+[target.'cfg(not(target_arch = "wasm32"))'.dependencies]
+rayon = { workspace = true }