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/step_in/brep.rs

@@ -395,12 +395,29 @@ pub(crate) fn read_item(cx: &Ctx, id: u64) -> R<BrepOut> {
     if shells.is_empty() {
         return Err(failed.first().cloned().unwrap_or_else(|| "no shells".into()));
     }
-    let solid = match guard("step solid", || mt::Solid::try_new(shells.clone()).map_err(|e| crate::KernelError::Failed(e.to_string()))) {
-        Ok(s) => s,
-        Err(_) => {
-            closed = false;
-            mt::Solid::new_unchecked(shells)
+    // `mt::Solid::try_new` only checks each shell (non-empty, connected, closed, no singular
+    // vertex) and hands the shells back unchanged (truck-topology 0.6.0), so the same checks run
+    // here on the borrowed shells. The successful path used to clone every face, edge, vertex,
+    // curve and surface to keep a copy that was then dropped.
+    let checked = guard("step solid", || {
+        for shell in &shells {
+            if shell.is_empty() {
+                return Err(crate::KernelError::Failed("empty shell".into()));
+            }
+            if !shell.is_connected() {
+                return Err(crate::KernelError::Failed("shell is not connected".into()));
+            }
+            if shell.shell_condition() != mt::ShellCondition::Closed {
+                return Err(crate::KernelError::Failed("shell is not closed".into()));
+            }
+            if !shell.singular_vertices().is_empty() {
+                return Err(crate::KernelError::Failed("shell is not manifold".into()));
+            }
         }
-    };
-    Ok(BrepOut { solid, failed, closed, file_faces, face_origin })
+        Ok(())
+    });
+    if checked.is_err() {
+        closed = false;
+    }
+    Ok(BrepOut { solid: mt::Solid::new_unchecked(shells), failed, closed, file_faces, face_origin })
 }