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/tests/CMakeLists.txt

@@ -58,10 +58,13 @@ find_package(Eigen3 REQUIRED)
 add_executable(test_federation
     test_federation.cpp
     ${IFCVIEWER_SRC}/Federation.cpp
-    # Federation pulls in Unit::convert for federation_unit_to_meters; compile
-    # Unit.cpp directly so the test doesn't have to link the whole IfcViewer
-    # library (which would drag in Qt6::OpenGL, OpenCASCADE, etc.).
+    # Federation pulls in Unit::convert for federationUnitToMeters and
+    # Geolocation helpers (helmertMetersFromParameters, getWcs, getMapUnit)
+    # for computeModelGeoref; compile both directly so the test doesn't
+    # have to link the whole IfcViewer library (which would drag in
+    # Qt6::OpenGL, OpenCASCADE, etc.).
     ${IFCVIEWER_SRC}/Unit.cpp
+    ${IFCVIEWER_SRC}/Geolocation.cpp
 )
 set_target_properties(test_federation PROPERTIES AUTOMOC ON)
 target_include_directories(test_federation PRIVATE ${IFCVIEWER_SRC})