ifcviewer: auto-guess FederatedFalseOrigin on first model added

When the user adds a model into a fresh, untitled federation that still
has the default (0,0,0, no rotation) FederatedFalseOrigin, derive an
origin from the first instance's placement_transformation (lifted
through CoordinateOperation when enabled) and the helmert grid-north
baked into ModelGeoref::coordinate_operation_meters.  Multi-file batches
naturally settle: whichever load finishes first anchors the federation,
the rest see a non-default origin and skip.  Saved .ifcfeds keep their
authoritative origin.

Adds Placement.{h,cpp} (port of util/placement.py — a2p,
get_axis2placement, get_local_placement) so Geolocation no longer needs
its own anonymous getAxis2Placement, and xaxis2angleDeg in Geolocation
mirroring util/geolocation.xaxis2angle.

SceneLoader captures the first instance's placement_transformation from
either the sidecar's InstanceCpu[0] or the streamer's first
InstanceChunk, so the guess works on both load paths without re-reading
the IFC.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ifcviewer/Placement.h

@@ -0,0 +1,50 @@
+/********************************************************************************
+ *                                                                              *
+ * This file is part of IfcOpenShell.                                           *
+ *                                                                              *
+ * IfcOpenShell is free software: you can redistribute it and/or modify         *
+ * it under the terms of the Lesser GNU General Public License as published by  *
+ * the Free Software Foundation, either version 3.0 of the License, or          *
+ * (at your option) any later version.                                          *
+ *                                                                              *
+ * IfcOpenShell is distributed in the hope that it will be useful,              *
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of               *
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the                 *
+ * Lesser GNU General Public License for more details.                          *
+ *                                                                              *
+ * You should have received a copy of the Lesser GNU General Public License     *
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.         *
+ *                                                                              *
+ ********************************************************************************/
+
+// Port of selected helpers from
+// src/ifcopenshell-python/ifcopenshell/util/placement.py — entity-instance
+// placement chains -> 4x4 matrices, used by callers that need to read
+// IfcLocalPlacement / IfcAxis2Placement* outside of the geometry kernel.
+
+#ifndef PLACEMENT_H
+#define PLACEMENT_H
+
+#include "../ifcparse/express.h"
+
+#include <Eigen/Dense>
+
+// Build a 4x4 placement matrix from an origin + Z + X axis triple, mirroring
+// ifcopenshell.util.placement.a2p.  The Y axis is derived as Z × X.  Inputs
+// don't need to be unit; vectors are renormalised internally.
+Eigen::Matrix4d a2p(const Eigen::Vector3d& origin,
+                    const Eigen::Vector3d& z,
+                    const Eigen::Vector3d& x);
+
+// IfcAxis2Placement{2D,3D,Linear} / IfcAxis1Placement -> 4x4 matrix.  Mirrors
+// ifcopenshell.util.placement.get_axis2placement.  Returns identity for null
+// or unparseable inputs.  Translation is in the IFC's project length unit.
+Eigen::Matrix4d getAxis2Placement(const express::Base& placement);
+
+// Resolve an IfcLocalPlacement (or an IfcAxis2Placement* directly) into a
+// 4x4 matrix in the IFC project's length unit, walking the PlacementRelTo
+// chain.  Mirrors ifcopenshell.util.placement.get_local_placement.  Returns
+// identity for a null input.
+Eigen::Matrix4d getLocalPlacement(const express::Base& placement);
+
+#endif // PLACEMENT_H