ifcviewer: de-Qt math types (Eigen everywhere)

Replace Qt math wrappers with Eigen across ViewportWindow, OverlayRenderer,
Federation, and the bonsai-side viewport modules. Eigen was already the
canonical type for the actually-important matrix work (InstanceCompose,
ModelGpuData, federation matrices); QVector3D/QVector4D/QMatrix4x4 were
leftover from when Qt was the path of least resistance. They offered
nothing over Eigen for our use case beyond a few graphics helpers
(lookAt / perspective / ortho) which were 30 lines to write.

Substitutions:
  QMatrix4x4 → Eigen::Matrix4f
  QVector2D  → Eigen::Vector2f
  QVector3D  → Eigen::Vector3f
  QVector4D  → Eigen::Vector4f

API rewrites:
  .lengthSquared()         → .squaredNorm()
  .length()                → .norm()
  .isNull()                → .isZero()
  .setToIdentity()         → .setIdentity()
  .constData()             → .data()
  .toVector3D()            → .head<3>()
  .inverted(&ok)           → tryInvert4f(M, out)
  Q::dotProduct(a,b)       → a.dot(b)
  Q::crossProduct(a,b)     → a.cross(b)
  QMat4x4(... row-major)   → Eigen::Map<const Matrix4f>(col-major buf)
  QMat4x4().lookAt(...)    → lookAtRH(eye, target, up)
  QMat4x4().perspective(.) → perspectiveYFovGL(fovy, aspect, n, f)
  QMat4x4().ortho(...)     → orthoGL(l, r, b, t, n, f)

Default-init divergence handled explicitly (QMatrix4x4() = identity,
QVector3D() = zero; Eigen leaves both uninitialized). Public API
(CameraState, HomeView, ViewportWindow::computeObjectAabb, the
addSectionPlaneAtSurface / pickSurfaceAt / raycast signatures) follows
through to Eigen too; bonsai-side View.cpp and Commands.cpp updated to
match.

Camera helpers (lookAtRH, perspectiveYFovGL, orthoGL, tryInvert4f)
extracted to a new CameraMath.h so OverlayRenderer's gizmo MVP and
ViewportWindow's buildViewProj share the same definitions. Federation
drops its <QVector3D> include in favour of <Eigen/Dense> (already had
the latter for the georef matrices).

Builds: desktop IfcViewerMinimal ✓, BonsaiViewer ✓, web IfcViewerWeb ✓.
Tests: 100/100 pass. Closes #78 + #79; opens the door for #80-#83.

src/ifcviewer/Federation.cpp

@@ -719,9 +719,9 @@ bool Federation::load(const QString& path,
         QJsonArray ta = ho.value("target").toArray();
         HomeView v;
         if (ta.size() == 3) {
-            v.target = QVector3D(float(ta[0].toDouble()),
-                                 float(ta[1].toDouble()),
-                                 float(ta[2].toDouble()));
+            v.target = Eigen::Vector3f(float(ta[0].toDouble()),
+                                       float(ta[1].toDouble()),
+                                       float(ta[2].toDouble()));
         }
         v.distance = float(ho.value("distance").toDouble(50.0));
         v.yaw      = float(ho.value("yaw").toDouble(45.0));