ifcviewer: cache per-model georef in SceneLoader

Adds ModelGeoref { ModelUnits units; Eigen::Matrix4d stage2_meters; bool
has_stage2; } and computeModelGeoref(file*) in Federation.{h,cpp}.  The
helper reads the project length unit, IfcProjectedCRS.MapUnit, helmert
parameters and WCS, and reduces them to a metres-in/metres-out stage 2
matrix using the existing Geolocation + Unit primitives.  When the model
has no IfcMapConversion it returns an identity stage_2 with has_stage2
== false, so the upload pipeline can branch cheaply.

SceneLoader::Model gains a cached ModelGeoref; SceneLoader::modelGeoref
(uint32_t mid) computes lazily on first call (returns nullptr when the
IFC file isn't available yet — happens on the sidecar-hit path before
the data-source thread populates the streamer) and serves from cache
afterwards.

Not yet consumed by the upload pipeline; that's the next commit.

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

src/ifcviewer/SceneLoader.h

@@ -33,6 +33,7 @@
 #include <thread>
 #include <vector>
 
+#include "Federation.h"
 #include "ViewportWindow.h"
 #include "GeometryStreamer.h"
 #include "SidecarCache.h"
@@ -65,6 +66,12 @@ public:
     QString displayName(uint32_t mid) const;
     ifcopenshell::file* ifcFile(uint32_t mid) const;
 
+    // Lazily computes the model's georef matrix + unit scales the first
+    // time it's asked for, caches the result, and returns a pointer into the
+    // cache.  Returns nullptr when the IFC file isn't available yet (e.g.
+    // sidecar-hit path before the data-source thread populates the streamer).
+    const ModelGeoref* modelGeoref(uint32_t mid);
+
 signals:
     void progressChanged(int percent);
     void loadStarted(uint32_t mid, QString display_name);
@@ -113,6 +120,11 @@ private:
         QString display_name;
         GeometryStreamer* streamer = nullptr;
         QElapsedTimer load_timer;
+
+        // Cached on first SceneLoader::modelGeoref(mid) call once the
+        // streamer has its IFC file loaded.
+        ModelGeoref georef;
+        bool        has_georef = false;
     };
 
     void startNextLoad();