ifcviewer: add incremental orbit/pan/zoom to ViewportCore (#85)

The orbit navigation math lived in the Qt ViewportWindow, operating on
ViewportCore's camera fields through references. That left the web host
with no way to drive the camera — orbit/pan/zoom were Qt-only.

Lift the three pixel-delta moves into ViewportCore as orbitBy / panBy /
dollyBy so every host (Qt desktop + web) shares one implementation and
the math can't drift between platforms. panBy takes the viewport height
as a parameter (the one Qt coupling: pan's world-units-per-pixel needs
it) so the core stays toolkit-free.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ifcviewer/ViewportCore.cpp

@@ -445,6 +445,53 @@ void ViewportCore::setStandardView(float yaw_deg, float pitch_deg) {
     host_->requestFrame();
 }
 
+void ViewportCore::orbitBy(float dx_px, float dy_px) {
+    // 0.4 deg/px matches the GL viewport. pitch is clamped just shy of
+    // the pole so orbitEye() stays well-conditioned.
+    camera_yaw_deg_   -= dx_px * 0.4f;
+    camera_pitch_deg_ += dy_px * 0.4f;
+    camera_pitch_deg_ = std::clamp(camera_pitch_deg_, -89.9f, 89.9f);
+    host_->requestFrame();
+}
+
+void ViewportCore::panBy(float dx_px, float dy_px, int viewport_height_px) {
+    constexpr float kDeg2Rad = float(M_PI) / 180.0f;
+
+    // Pan in the camera's screen-space plane. Within 1° of straight
+    // up/down the world-Z up-reference degenerates (cross with forward
+    // is the zero vector → NaN), so switch to world-Y up — matches the
+    // up-vector switch in buildViewProj so top/bottom views still pan.
+    const Eigen::Vector3f target(camera_target_[0], camera_target_[1], camera_target_[2]);
+    const Eigen::Vector3f eye = orbitEye(camera_target_, camera_distance_,
+                                         camera_yaw_deg_, camera_pitch_deg_);
+    const Eigen::Vector3f fwd = (target - eye).normalized();
+    const Eigen::Vector3f world_up = (std::abs(camera_pitch_deg_) >= 89.0f)
+                                     ? Eigen::Vector3f(0.0f, 1.0f, 0.0f)
+                                     : Eigen::Vector3f(0.0f, 0.0f, 1.0f);
+    const Eigen::Vector3f right = fwd.cross(world_up).normalized();
+    const Eigen::Vector3f up    = right.cross(fwd).normalized();
+
+    const float half_h_world = camera_distance_
+        * std::tan(camera_fov_y_deg_ * 0.5f * kDeg2Rad);
+    const float pan_per_pixel = (viewport_height_px > 0)
+        ? (2.0f * half_h_world / float(viewport_height_px))
+        : 0.0f;
+
+    const Eigen::Vector3f shift = -right * (dx_px * pan_per_pixel)
+                                +  up    * (dy_px * pan_per_pixel);
+    camera_target_[0] += shift.x();
+    camera_target_[1] += shift.y();
+    camera_target_[2] += shift.z();
+    host_->requestFrame();
+}
+
+void ViewportCore::dollyBy(float notches) {
+    // Each notch zooms ~10% in/out; sign matches "wheel up = in".
+    const float factor = std::pow(0.9f, notches);
+    camera_distance_   = std::max(0.01f, camera_distance_ * factor);
+    host_->requestFrame();
+}
+
 void ViewportCore::toggleProjection() {
     projection_ortho_ = !projection_ortho_;
     std::fprintf(stderr, "[info] [wgpu] projection: %s\n",