cimery-ifc README + usd 테스트 unused import 정리

## crates/ifc/README.md 신설
- Phase 1~3c 현황표 (Sprint 33~37) + 미구현 로드맵
- 사용 예: 직접 구성 / viewer scene_params_to_ifc 변환
- IFC 뷰어 호환성 (BlenderBIM/Solibri/Navisworks/IfcOpenShell)
- GUID 설명, snapshot 테스트 가이드

## crates/usd/src/lib.rs
cargo fix 로 unused imports 자동 정리 (test 모듈).

cargo check --workspace --all-targets --features occt: 0 경고.

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

cimery/crates/usd/src/lib.rs

@@ -288,10 +288,10 @@ impl BridgeExporter {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use cimery_core::{AbutmentType, BearingType, MaterialGrade, PierType, ColumnShape, SectionType};
+    use cimery_core::{MaterialGrade, SectionType};
     use cimery_ir::{
-        AbutmentIR, BearingIR, CapBeamIR, FeatureId, GirderIR,
-        PscISectionParams, SectionParams, WingWallIR,
+        FeatureId, GirderIR,
+        PscISectionParams, SectionParams,
     };
     use cimery_kernel::{PureRustKernel, StubKernel};