Stop mutating dirLocation while importing CAD files

The STL import loop used dirLocation.append("/").append(cf), which
mutates dirLocation in place. After the first file, dirLocation was no
longer the directory path — the second iteration produced paths like
.../dir/a.stl/b.stl, so only the first STL resolved. The mangled value
also leaked to line 33, where the per-variation cad__<variation>.yaml
modifier path is built, so that override file was never found.

Use non-mutating concatenation (dirLocation + "/" + cf) so dirLocation
stays the directory path for every file and for the YAML lookup.

Fixes #110

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: zhaozhiwen

gemc/gsystem/gsystemFactories/cad/loadGeometry.cc

@@ -26,7 +26,7 @@ void GSystemCADFactory::loadGeometry(GSystem* s) {
 
 		// Import each STL as a volume. Each volume name is derived from the filename.
 		for (const auto& cf : cadFiles) {
-			s->addVolumeFromFile(GSYSTEMCADTFACTORYLABEL, dirLocation.append("/").append(cf));
+			s->addVolumeFromFile(GSYSTEMCADTFACTORYLABEL, dirLocation + "/" + cf);
 		}
 
 		// If the file cad__<variation>.yaml is found in dirLocation, modify the gvolumes accordingly.