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-full/CMakeLists.txt

@@ -34,7 +34,3 @@ set_target_properties(IfcViewerFull PROPERTIES
 target_link_libraries(IfcViewerFull PRIVATE IfcViewer)
 
 install(TARGETS IfcViewerFull EXPORT ${IFCOPENSHELL_EXPORT_TARGETS})
-
-if(BUILD_IFCVIEWER_TESTS)
-    add_subdirectory(tests)
-endif()