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.h

@@ -72,6 +72,14 @@ public:
     // sidecar-hit path before the data-source thread populates the streamer).
     const ModelGeoref* modelGeoref(uint32_t mid);
 
+    // The placement_transformation (in metres, column-major 4x4) of the
+    // first instance the loader saw for `mid` — captured from the streamer's
+    // first InstanceChunk during a stream load, or from the cached
+    // InstanceCpu[0] on a sidecar hit.  Returns nullptr until at least one
+    // instance has been observed.  Used by the federation false-origin
+    // auto-guess to anchor the model without re-parsing the IFC.
+    const Eigen::Matrix4d* firstPlacement(uint32_t mid) const;
+
 signals:
     void progressChanged(int percent);
     void loadStarted(uint32_t mid, QString display_name);
@@ -125,6 +133,12 @@ private:
         // streamer has its IFC file loaded.
         ModelGeoref georef;
         bool        has_georef = false;
+
+        // The first instance's placement_transformation (in metres) — set
+        // once per model from either the sidecar's InstanceCpu[0] or the
+        // streamer's first InstanceChunk.
+        Eigen::Matrix4d first_placement     = Eigen::Matrix4d::Identity();
+        bool            has_first_placement = false;
     };
 
     void startNextLoad();