ifcviewer: move camera math into ViewportCore (#84-h)

Move the three camera-math methods that compute view/projection
matrices, scene bounds, and per-chunk screen footprint for the
streaming priority signal:

  void  buildViewProj(Eigen::Matrix4f&, Eigen::Matrix4f&) const
  bool  computeSceneAabb(float[3], float[3]) const
  float chunkScreenAreaPx(const ModelGpuData::Chunk&,
                          const Eigen::Matrix4f&) const

Plus the orbitEye helper (anonymous namespace in ViewportCore.cpp;
the qDegreesToRadians dep got swapped for an inline M_PI/180 constant).

ViewportWindow.cpp's 9 internal callers (cull, streaming, pick,
render, debug) updated to use core_.buildViewProj() etc. The
buildViewProj forwarder stays out of ViewportWindow.h since no
external caller needs it — bonsai/minimal both go through
public API methods like viewAll which still wrap core_ access
on the VW side.

Builds: desktop / bonsai / web all green. Tests 100/100.

src/ifcviewer/ViewportCore.h

@@ -109,6 +109,21 @@ public:
     // completed.
     void recomposeAndUploadModel(uint32_t model_id);
 
+    // ---- Camera math --------------------------------------------------------
+    //
+    // buildViewProj feeds every cull, streaming, pick and render path
+    // — keep it as a single helper so the projection_ortho_ toggle
+    // and the near-vertical up-vector switch can't drift between
+    // call sites. computeSceneAabb folds every visible model's
+    // world AABBs into one — used by viewAll and the bench camera.
+    // chunkScreenAreaPx projects one chunk's world AABB through a
+    // VP into 2D pixels — the streaming loader's priority signal.
+    void buildViewProj(Eigen::Matrix4f& view_out,
+                       Eigen::Matrix4f& proj_out) const;
+    bool computeSceneAabb(float mn[3], float mx[3]) const;
+    float chunkScreenAreaPx(const ModelGpuData::Chunk& c,
+                            const Eigen::Matrix4f& vp_mat) const;
+
     // Friend access for ViewportWindow's reference proxies. As each
     // render method moves into ViewportCore it stops needing these
     // (it touches the fields directly); once everything has migrated