perf: prism pairs before the mesh, bodies measured once, and a STEP import that copies less (#714)

A cut or an intersection between prisms (a plate and the tools through it) is now decided in 2D before either operand is meshed. A body's size and volume are measured once, a placement reads its orientation from its matrix, the evaluation cache is bounded to 64 MiB, and STEP import copies fewer shells. Hole plates evaluate 20-30% faster with less memory, and models are unchanged.

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

crates/kernel/src/surfaces.rs

@@ -31,6 +31,8 @@ pub(crate) fn surface_body(faces: Vec<mt::Face>) -> Body {
 fn surface_of(shell: mt::Shell) -> Body {
     Body {
         solid: std::sync::Arc::new(mt::Solid::new_unchecked(vec![shell])),
+        size: 0.0,
+        volume: Some(std::sync::Arc::new(std::sync::Mutex::new(None))),
         coarse_missing: None,
         mesh: None,
         color: None,