ifcviewer: merge Federation classes into the lib

Move src/ifcviewer-full/Federation.{h,cpp} (and its tests) into
src/ifcviewer/ so the lib stays the single source of truth for the
federation data model.  Restores the original "agnostic lib usable
from ifcviewer-full and ifcviewer-minimal alike" framing.

Drop the unused per-model transform[16] / has_transform field — it
was round-trip-only with no UI to author it, and is being replaced
by an intent-based ModelTransform in the next commit.  No real
.ifcfed in the wild populated this field; old files still load
(unknown JSON keys ignored), they just lose the unused transform.

Replaces the pure-data-model Federation.{h,cpp} that was added a
few commits earlier — that file's structs and compose helpers
return as part of the merged Federation in commit 6.

ifcviewer-full's per-app tests dir is removed (test_federation was
the only one); BUILD_IFCVIEWER_TESTS now wires test_federation in
under src/ifcviewer/tests/, with the Qt6::Core/Gui/Test dependency
declared inline since unlike the other Tier-1 tests it has to pull
Qt in.  All 31 tests pass.

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

src/ifcviewer/tests/CMakeLists.txt

@@ -47,3 +47,22 @@ add_ifcviewer_unit_test(test_sidecar_cache
 )
 
 add_ifcviewer_unit_test(test_instanced_geometry)
+
+# Federation is Qt-derived (QObject + signals + QVector3D + QJson*).  Unlike
+# the other Tier-1 tests it has to pull Qt6::Core/Gui/Test in directly and
+# enable AUTOMOC for the Q_OBJECT moc-generation.
+find_package(Qt${QT_VERSION} COMPONENTS Core Gui Test REQUIRED PATHS ${QT_DIR})
+
+add_executable(test_federation
+    test_federation.cpp
+    ${IFCVIEWER_SRC}/Federation.cpp
+)
+set_target_properties(test_federation PROPERTIES AUTOMOC ON)
+target_include_directories(test_federation PRIVATE ${IFCVIEWER_SRC})
+target_link_libraries(test_federation PRIVATE
+    Catch2::Catch2WithMain
+    Qt${QT_VERSION}::Core
+    Qt${QT_VERSION}::Gui    # Federation::HomeView uses QVector3D from QtGui
+    Qt${QT_VERSION}::Test   # QSignalSpy
+)
+catch_discover_tests(test_federation)