cimery Sprint 2 — PSC-I 기하학 + viewer 개편 + OCCT optional

kernel:
- PureRustKernel: PSC-I 단면 14-vertex polygon 스위프, flat normals
  56 triangles / 168 vertices, 법선 단위벡터 검증 포함
- opencascade 의존성 optional feature (--features occt)로 격리
  → OCCT 없이도 전체 빌드 가능
- psc_i.rs: 프로파일 검증, AABB, 법선 테스트 6개

viewer:
- camera.rs: arcball orbit (middle-mouse drag + scroll zoom)
- shader.wgsl: MVP matrix uniform + 방향성 조명 (콘크리트 베이지)
- lib.rs: depth buffer, index 렌더, 실제 Mesh 업로드
  StubKernel → PureRustKernel → OcctKernel 교체 경로 문서화

CLAUDE.md: MVP 품질 원칙 강화 ("아키텍처 임의 변경 절대 불가")

cargo test --workspace (viewer 제외) 43개 전부 통과

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

cimery/crates/viewer/src/camera.rs

@@ -0,0 +1,101 @@
+//! Arcball/orbit camera — Revit ViewCube style.
+//!
+//! Orbit with middle-mouse drag, zoom with scroll wheel.
+
+use glam::{Mat4, Vec3};
+use bytemuck::{Pod, Zeroable};
+
+// ─── GPU uniform ─────────────────────────────────────────────────────────────
+
+/// 64-byte view-projection matrix uploaded to the GPU once per frame.
+#[repr(C)]
+#[derive(Copy, Clone, Debug, Pod, Zeroable)]
+pub struct CameraUniform {
+    pub view_proj: [[f32; 4]; 4],
+}
+
+impl CameraUniform {
+    pub fn identity() -> Self {
+        Self { view_proj: Mat4::IDENTITY.to_cols_array_2d() }
+    }
+}
+
+// ─── Camera ──────────────────────────────────────────────────────────────────
+
+/// Orbit camera — spherical coordinates around a fixed target point.
+///
+/// All distances in millimetres (scene units).
+pub struct Camera {
+    /// Point the camera orbits around.
+    pub target:  Vec3,
+    /// Distance from target [mm].
+    pub radius:  f32,
+    /// Horizontal rotation [radians].
+    pub yaw:     f32,
+    /// Vertical rotation [radians]. Clamped to avoid gimbal lock.
+    pub pitch:   f32,
+    pub fov_y:   f32,
+    pub aspect:  f32,
+    pub znear:   f32,
+    pub zfar:    f32,
+}
+
+impl Camera {
+    /// Default view looking at a 40 m PSC-I girder from a comfortable angle.
+    pub fn default_for_girder(span_mm: f32) -> Self {
+        Self {
+            target:  Vec3::new(300.0, 900.0, span_mm * 0.5),
+            radius:  span_mm * 1.5,
+            yaw:     std::f32::consts::FRAC_PI_4,   // 45°
+            pitch:   0.35,                           // ~20°
+            fov_y:   60.0_f32.to_radians(),
+            aspect:  16.0 / 9.0,
+            znear:   10.0,                           // 10 mm
+            zfar:    10_000_000.0,                   // 10 km
+        }
+    }
+
+    /// Eye position derived from orbit parameters.
+    pub fn eye(&self) -> Vec3 {
+        let sin_p = self.pitch.sin();
+        let cos_p = self.pitch.cos();
+        let sin_y = self.yaw.sin();
+        let cos_y = self.yaw.cos();
+        self.target + Vec3::new(
+            self.radius * cos_p * sin_y,
+            self.radius * sin_p,
+            self.radius * cos_p * cos_y,
+        )
+    }
+
+    /// View-projection matrix (right-handed, depth 0→1).
+    pub fn view_proj(&self) -> Mat4 {
+        let view = Mat4::look_at_rh(self.eye(), self.target, Vec3::Y);
+        let proj = Mat4::perspective_rh(self.fov_y, self.aspect, self.znear, self.zfar);
+        proj * view
+    }
+
+    /// Build GPU uniform from current state.
+    pub fn to_uniform(&self) -> CameraUniform {
+        CameraUniform { view_proj: self.view_proj().to_cols_array_2d() }
+    }
+
+    // ── Interaction ────────────────────────────────────────────────────────
+
+    /// Orbit by dragging (delta in pixels, scaled to radians).
+    pub fn orbit(&mut self, delta_x: f32, delta_y: f32) {
+        self.yaw   += delta_x * 0.005;
+        self.pitch  = (self.pitch - delta_y * 0.005)
+            .clamp(-std::f32::consts::FRAC_PI_2 + 0.05, std::f32::consts::FRAC_PI_2 - 0.05);
+    }
+
+    /// Zoom by scrolling (positive = closer, negative = farther).
+    pub fn zoom(&mut self, delta: f32) {
+        self.radius = (self.radius * (1.0 - delta * 0.1)).max(100.0);
+    }
+
+    /// Update aspect ratio on window resize.
+    pub fn resize(&mut self, width: u32, height: u32) {
+        self.aspect = width as f32 / height.max(1) as f32;
+    }
+}