Sprint 35 — 뷰어 IFC 익스포트 연결 + Pset_BeamCommon

## 뷰어 통합
- `cimery-viewer` → `cimery-ifc` 의존성 추가.
- `project_file::scene_params_to_ifc()` 변환 함수:
  SceneParams 의 모든 파라미터(경간 수·교각 형식·skew·헌치·단면 종류·신축이음)
  를 BridgeExportParams 로 전부 매핑.
- egui 프로젝트 섹션에 "📤 IFC4X3 익스포트" 버튼.
  현재 파라미터 상태로 즉시 `projects/bridge.ifc` 생성.
- `project_file::default_ifc_path()` 헬퍼.

## Pset_BeamCommon (IFC Phase 3a)
- `write_pset_beam_common()`: 4개 속성
  · Reference (IFCIDENTIFIER) — 거더 라벨
  · Span (IFCLENGTHMEASURE) — mm
  · LoadBearing (IFCBOOLEAN) — .T.
  · IsExternal (IFCBOOLEAN) — .F.
- IFCRELDEFINESBYPROPERTIES 로 각 IFCBEAM 에 연결.
- `IfcSectionKind` public re-export (viewer 에서 직접 참조).

## 테스트
- pset_beam_common_attached_to_girders 추가. 17개 전체 통과.
- cargo check --workspace --features occt: 0 errors.

Phase 3 남은 로드맵:
- IfcAlignment + IfcLinearPlacement
- Camber 반영 (현재 직선 girder 만)
- Pset_BearingCommon, Pset_SlabCommon
- IfcElementAssembly 로 Pier(column+capbeam) 그룹화

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

cimery/crates/viewer/src/project_file.rs

@@ -131,3 +131,37 @@ pub fn default_save_path(name: &str) -> std::path::PathBuf {
     p.push(format!("{}.cimery.json", name));
     p
 }
+
+/// IFC 파일 기본 저장 경로.
+pub fn default_ifc_path(name: &str) -> std::path::PathBuf {
+    let mut p = std::path::PathBuf::from("projects");
+    std::fs::create_dir_all(&p).ok();
+    p.push(format!("{}.ifc", name));
+    p
+}
+
+/// SceneParams → cimery-ifc::BridgeExportParams 변환.
+///
+/// viewer 의 SceneParams 에 있는 모든 파라미터를 IFC 익스포터 입력으로 매핑.
+/// 선형(alignment)·camber 는 IFC Phase 3 로드맵(미반영).
+pub fn scene_params_to_ifc(p: &SceneParams, name: &str) -> cimery_ifc::BridgeExportParams {
+    use cimery_ifc::{BridgeExportParams, IfcSectionKind};
+    BridgeExportParams {
+        name:           name.to_owned(),
+        span_m:         p.span_m,
+        span_count:     p.span_count,
+        girder_count:   p.girder_count,
+        girder_spacing: p.girder_spacing as f64,
+        girder_height:  p.girder_height as f64,
+        slab_thickness: p.slab_thickness as f64,
+        bearing_height: 60.0,
+        section_kind:   match p.section_type {
+            GirderSectionType::PscI     => IfcSectionKind::PscI,
+            GirderSectionType::SteelBox => IfcSectionKind::SteelBox,
+        },
+        skew_deg:       p.skew_deg as f64,
+        haunch_depth:   p.haunch_depth as f64,
+        show_parapets:  true,
+        show_joints:    p.show_expansion_joints,
+    }
+}