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/SceneLoader.cpp

@@ -81,6 +81,12 @@ const ModelGeoref* SceneLoader::modelGeoref(uint32_t mid) {
     return &m.georef;
 }
 
+const Eigen::Matrix4d* SceneLoader::firstPlacement(uint32_t mid) const {
+    auto it = models_.find(mid);
+    if (it == models_.end() || !it->second.has_first_placement) return nullptr;
+    return &it->second.first_placement;
+}
+
 std::vector<uint32_t> SceneLoader::addFiles(const QStringList& paths) {
     std::vector<uint32_t> assigned;
     assigned.reserve(paths.size());
@@ -214,6 +220,14 @@ void SceneLoader::applySidecarData(uint32_t mid, SidecarData data) {
         model.has_georef                   = true;
     }
 
+    if (!data.instances.empty() && !model.has_first_placement) {
+        using Mat4fCol = Eigen::Matrix<float, 4, 4, Eigen::ColMajor>;
+        model.first_placement =
+            Eigen::Map<const Mat4fCol>(data.instances[0].placement_transformation)
+                .cast<double>();
+        model.has_first_placement = true;
+    }
+
     std::vector<PackedElementInfo> elements = std::move(data.elements);
     std::string stbl                        = std::move(data.string_table);
 
@@ -302,6 +316,15 @@ void SceneLoader::onStreamerMeshReady(MeshChunk chunk) {
 }
 
 void SceneLoader::onStreamerInstanceReady(InstanceChunk chunk) {
+    if (loading_model_id_ != 0) {
+        auto it = models_.find(loading_model_id_);
+        if (it != models_.end() && !it->second.has_first_placement) {
+            using Mat4fCol = Eigen::Matrix<float, 4, 4, Eigen::ColMajor>;
+            it->second.first_placement =
+                Eigen::Map<const Mat4fCol>(chunk.transform).cast<double>();
+            it->second.has_first_placement = true;
+        }
+    }
     viewport_->uploadInstanceChunk(chunk);
 }