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/OverlayRenderer.h

@@ -21,10 +21,10 @@
 #define WGPUOVERLAYRENDERER_H
 
 #include <QHash>
-#include <QMatrix4x4>
 #include <QPoint>
 #include <QString>
-#include <QVector3D>
+
+#include <Eigen/Dense>
 
 #include <webgpu/webgpu.h>
 
@@ -35,8 +35,8 @@
 // at the top of render() and passed by const-ref to each encodeX() call so
 // the overlay renderer never reaches back into the viewport.
 struct OverlayFrame {
-    QMatrix4x4 view_proj;
-    QVector3D  camera_target;
+    Eigen::Matrix4f view_proj      = Eigen::Matrix4f::Identity();
+    Eigen::Vector3f camera_target  = Eigen::Vector3f::Zero();
     float      camera_distance     = 5.0f;
     float      camera_yaw_deg      = 0.0f;
     float      camera_pitch_deg    = 0.0f;
@@ -51,12 +51,11 @@ struct OverlayFrame {
 // reads from a non-owning span every frame. Held by value because the
 // struct is small and copies happen at most six times per frame.
 struct SectionPlane {
-    QVector3D n;             // unit normal (camera-facing after auto-flip)
-    float     d;             // -dot(n, origin)
-    QVector3D origin;        // surface point at the moment the plane was added
-    float     visual_radius; // unused by the visualizer (kept here so the
-                             // section tool's state struct round-trips
-                             // through this overlay-facing definition).
+    Eigen::Vector3f n      = Eigen::Vector3f::UnitZ(); // unit normal
+    float           d      = 0.0f;                     // -dot(n, origin)
+    Eigen::Vector3f origin = Eigen::Vector3f::Zero();  // surface point at the
+                                                       // moment the plane was added
+    float    visual_radius = 0.0f;
 };
 
 // All viewport overlays in one place: axis indicator (corner + pivot),