viewer: --features occt 시 OcctKernel B-rep 렌더링 연결

- viewer/Cargo.toml: occt feature → cimery-kernel/occt 전파
- lib.rs: cfg(feature = "occt") 분기
  - OcctKernel: OCCT B-rep PSC-I + BRepMesh 테셀레이션
  - PureRustKernel: 기본값 (occt 없을 때)
- 타이틀 자동: "OcctKernel B-rep" vs "PSC-I PureRustKernel"

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cimery/crates/viewer/Cargo.toml

@@ -3,6 +3,11 @@ name    = "cimery-viewer"
 version.workspace = true
 edition.workspace = true
 
+[features]
+# Enable OcctKernel (requires OCCT — see cimery/CLAUDE.md).
+# Build: cargo run -p cimery-viewer --features occt
+occt = ["cimery-kernel/occt"]
+
 [[bin]]
 name = "cimery-viewer"
 path = "src/main.rs"

cimery/crates/viewer/src/lib.rs

@@ -25,7 +25,11 @@ use winit::{
 use wgpu::util::DeviceExt;
 use cimery_core::{MaterialGrade, SectionType};
 use cimery_ir::{FeatureId, GirderIR, PscISectionParams, SectionParams};
-use cimery_kernel::{GeomKernel, PureRustKernel};
+#[cfg(feature = "occt")]
+use cimery_kernel::OcctKernel;
+#[cfg(not(feature = "occt"))]
+use cimery_kernel::PureRustKernel;
+use cimery_kernel::GeomKernel;
 use camera::Camera;
 
 // ─── Vertex ───────────────────────────────────────────────────────────────────
@@ -148,7 +152,9 @@ impl RenderState {
             spacing:  0.0,
             material: MaterialGrade::C50,
         };
-        // Sprint 3: replace PureRustKernel with OcctKernel (--features occt)
+        #[cfg(feature = "occt")]
+        let mesh = OcctKernel.girder_mesh(&test_ir).expect("OcctKernel mesh");
+        #[cfg(not(feature = "occt"))]
         let mesh = PureRustKernel.girder_mesh(&test_ir).expect("PureRustKernel mesh");
 
         let verts: Vec<Vertex> = mesh.vertices.iter().zip(mesh.normals.iter())
@@ -377,7 +383,11 @@ impl Default for CimeryApp {
 impl ApplicationHandler for CimeryApp {
     fn resumed(&mut self, event_loop: &ActiveEventLoop) {
         let attrs = Window::default_attributes()
-            .with_title("cimery viewer [Sprint 2 — PSC-I PureRustKernel]")
+            .with_title(if cfg!(feature = "occt") {
+                "cimery viewer [Sprint 3 — OcctKernel B-rep]"
+            } else {
+                "cimery viewer [Sprint 2 — PSC-I PureRustKernel]"
+            })
             .with_inner_size(winit::dpi::LogicalSize::new(1280u32, 720u32));
         let window = Arc::new(
             event_loop.create_window(attrs).expect("create window"),