OcctKernel 구현 — PSC-I B-rep sweep + BRepMesh 테셀레이션

- cimery-kernel/src/occt.rs: OcctKernel (--features occt 전용)
  - Workplane::xy().sketch()로 PSC-I 14-vertex 2D 프로파일 생성
  - Face::extrude(DVec3) → Solid (OCCT B-rep)
  - Mesher::try_new() + mesh() → 테셀레이션
  - cimery Mesh로 변환 (vertices/normals/indices)
  - 기타 Feature: PureRustKernel 위임
- kernel/Cargo.toml: glam 0.24 (opencascade-rs 동일 버전)
- cargo build --features occt 빌드 확인 완료

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

cimery/crates/kernel/src/occt.rs

@@ -0,0 +1,180 @@
+//! OcctKernel — geometry backend using OpenCASCADE Technology (OCCT).
+//!
+//! Only compiled with `--features occt` (ADR-001, ADR-002 optional feature).
+//!
+//! Produces proper B-rep solids:
+//! - Accurate PSC-I cross-section with haunches
+//! - Higher-quality tessellation (BRepMesh_IncrementalMesh)
+//! - Foundation for fillets, boolean ops in later sprints
+
+#[cfg(feature = "occt")]
+mod inner {
+    use glam::DVec3;
+    use opencascade::{
+        mesh::Mesher,
+        workplane::Workplane,
+    };
+    use cimery_ir::{
+        AbutmentIR, BearingIR, DeckSlabIR, GirderIR, PierIR,
+        PscISectionParams, SectionParams,
+    };
+    use crate::{KernelError, Mesh};
+
+    // ── Girder (PSC-I B-rep sweep) ──────────────────────────────────────────────
+
+    pub fn girder_mesh(ir: &GirderIR) -> Result<Mesh, KernelError> {
+        if ir.span_m() <= 0.0 {
+            return Err(KernelError::InvalidInput(
+                format!("span must be positive, got {} m", ir.span_m()),
+            ));
+        }
+        match &ir.section {
+            SectionParams::PscI(p)  => psc_i_extrude(p, ir.span_mm()),
+            // Other sections: fall back to PureRustKernel until implemented
+            _ => {
+                log::warn!(
+                    "OcctKernel: section {:?} not yet implemented, using PureRust fallback",
+                    ir.section_type
+                );
+                crate::psc_i::build_psc_i_mesh(
+                    match &ir.section {
+                        SectionParams::PscI(p) => p,
+                        _ => unreachable!(),
+                    },
+                    ir.span_mm(),
+                )
+            }
+        }
+    }
+
+    fn psc_i_extrude(p: &PscISectionParams, span_mm: f64) -> Result<Mesh, KernelError> {
+        let hw   = p.top_flange_width    / 2.0;
+        let hbw  = p.bottom_flange_width / 2.0;
+        let hwb  = p.web_thickness       / 2.0;
+        let h    = p.total_height;
+        let tft  = p.top_flange_thickness;
+        let bft  = p.bottom_flange_thickness;
+        let hch  = p.haunch;
+
+        // PSC-I 14-vertex profile in XY plane (mm), CCW
+        let wire = Workplane::xy()
+            .sketch()
+            .move_to(-hbw,             0.0          )
+            .line_to( hbw,             0.0          )
+            .line_to( hbw,             bft          )
+            .line_to( hwb,             bft          )
+            .line_to( hwb,         h - tft - hch    )
+            .line_to( hwb + hch,   h - tft          )
+            .line_to( hw,          h - tft          )
+            .line_to( hw,          h                )
+            .line_to(-hw,          h                )
+            .line_to(-hw,          h - tft          )
+            .line_to(-(hwb + hch), h - tft          )
+            .line_to(-hwb,         h - tft - hch    )
+            .line_to(-hwb,             bft          )
+            .line_to(-hbw,             bft          )
+            .close()
+            .to_face();
+
+        // Extrude along Z axis by span_mm
+        let solid = wire.extrude(DVec3::new(0.0, 0.0, span_mm));
+
+        // Tessellate with engineering-grade tolerance (1 mm)
+        let shape = solid.into();
+        let occt_mesh = Mesher::try_new(&shape, 1.0)
+            .map_err(|e| KernelError::Computation(format!("OCCT mesher init: {e}")))?
+            .mesh()
+            .map_err(|e| KernelError::Computation(format!("OCCT tessellation: {e}")))?;
+
+        occt_mesh_to_cimery(occt_mesh)
+    }
+
+    // ── Other features (use PureRustKernel geometry for now) ─────────────────────
+
+    pub fn deck_slab_mesh(ir: &DeckSlabIR) -> Result<Mesh, KernelError> {
+        crate::deck_slab::build_deck_slab_mesh(ir)
+    }
+
+    pub fn bearing_mesh(ir: &BearingIR) -> Result<Mesh, KernelError> {
+        crate::bearing::build_bearing_mesh(ir)
+    }
+
+    pub fn pier_mesh(ir: &PierIR) -> Result<Mesh, KernelError> {
+        crate::pier::build_pier_mesh(ir)
+    }
+
+    pub fn abutment_mesh(ir: &AbutmentIR) -> Result<Mesh, KernelError> {
+        crate::abutment::build_abutment_mesh(ir)
+    }
+
+    // ── Conversion ────────────────────────────────────────────────────────────────
+
+    fn occt_mesh_to_cimery(m: opencascade::mesh::Mesh) -> Result<Mesh, KernelError> {
+        if m.vertices.is_empty() {
+            return Err(KernelError::Computation(
+                "OCCT tessellation produced no vertices".into(),
+            ));
+        }
+
+        let vertices: Vec<[f32; 3]> = m.vertices.iter()
+            .map(|v| [v.x as f32, v.y as f32, v.z as f32])
+            .collect();
+
+        // normals array may be shorter (known OCCT API quirk, see mesh.rs)
+        let normals: Vec<[f32; 3]> = if m.normals.len() == vertices.len() {
+            m.normals.iter()
+                .map(|n| {
+                    let len = (n.x*n.x + n.y*n.y + n.z*n.z).sqrt();
+                    if len < 1e-10 {
+                        [0.0, 1.0, 0.0]
+                    } else {
+                        [(n.x/len) as f32, (n.y/len) as f32, (n.z/len) as f32]
+                    }
+                })
+                .collect()
+        } else {
+            // Fallback: flat normals per vertex (less ideal but correct)
+            vec![[0.0_f32, 1.0, 0.0]; vertices.len()]
+        };
+
+        let indices: Vec<u32> = m.indices.iter()
+            .map(|&i| i as u32)
+            .collect();
+
+        Ok(Mesh { vertices, normals, indices })
+    }
+}
+
+// ── Public struct (only when feature is active) ────────────────────────────────
+
+#[cfg(feature = "occt")]
+pub use self::occt_kernel::OcctKernel;
+
+#[cfg(feature = "occt")]
+mod occt_kernel {
+    use cimery_ir::{AbutmentIR, BearingIR, DeckSlabIR, GirderIR, PierIR};
+    use crate::{GeomKernel, KernelError, Mesh};
+    use super::inner;
+
+    /// Full B-rep geometry backend using OpenCASCADE Technology.
+    /// Enabled with `--features occt`. See ADR-001.
+    pub struct OcctKernel;
+
+    impl GeomKernel for OcctKernel {
+        fn girder_mesh(&self, ir: &GirderIR) -> Result<Mesh, KernelError> {
+            inner::girder_mesh(ir)
+        }
+        fn deck_slab_mesh(&self, ir: &DeckSlabIR) -> Result<Mesh, KernelError> {
+            inner::deck_slab_mesh(ir)
+        }
+        fn bearing_mesh(&self, ir: &BearingIR) -> Result<Mesh, KernelError> {
+            inner::bearing_mesh(ir)
+        }
+        fn pier_mesh(&self, ir: &PierIR) -> Result<Mesh, KernelError> {
+            inner::pier_mesh(ir)
+        }
+        fn abutment_mesh(&self, ir: &AbutmentIR) -> Result<Mesh, KernelError> {
+            inner::abutment_mesh(ir)
+        }
+    }
+}