Multi-model project support with sequential loading

Introduce ModelHandle and per-model GeometryStreamers so multiple IFC
files can be loaded simultaneously. Object IDs are globally unique
(monotonically increasing across models). File picker is now multiselect.
Each model gets a top-level tree node. Property lookup uses the correct
model's ifcopenshell::file. ViewportWindow supports hide/show/remove
per model via model_id filtering in the frustum cull pass.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ifcviewer/main.cpp

@@ -40,7 +40,7 @@ int main(int argc, char* argv[]) {
     QCommandLineParser parser;
     parser.setApplicationDescription("IfcOpenShell IFC Viewer");
     parser.addHelpOption();
-    parser.addPositionalArgument("file", "IFC file to open");
+    parser.addPositionalArgument("files", "IFC file(s) to open", "[files...]");
     parser.process(app);
 
     MainWindow window;
@@ -48,7 +48,7 @@ int main(int argc, char* argv[]) {
 
     auto args = parser.positionalArguments();
     if (!args.isEmpty()) {
-        window.openFile(args.first());
+        window.addFiles(args);
     }
 
     return app.exec();