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"