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/native.rs

@@ -84,6 +84,8 @@ pub fn decode_native_body(bytes: &[u8]) -> Result<Body> {
         let solid = adapter::decode(saved.solid)?;
         let mut body = Body {
             solid: Arc::new(solid),
+            size: 0.0,
+            volume: Some(std::sync::Arc::new(std::sync::Mutex::new(None))),
             coarse_missing: None,
             mesh: None,
             color: None,