fix: occt.rs unused_mut 경고 제거

breast wall 메시 변수를 추가 수정 없이 바로 parts 에 push 하므로 mut 불필요.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cimery/crates/kernel/src/occt.rs

@@ -179,7 +179,7 @@ mod inner {
             let hw  = bww * 0.5;
             let hbt = bwt * 0.5;
             let profile = vec![(-hw, -hbt), (hw, -hbt), (hw, hbt), (-hw, hbt)];
-            let mut m = workplane_extrude_xz_y(&profile, bwh)?;
+            let m = workplane_extrude_xz_y(&profile, bwh)?;
             // Breast wall at footing top, Y=0..bwh
             parts.push(m);
         }