ifcviewer: key sidecar on path stem, drop staleness check

Previously readSidecar/writeSidecar were keyed on (path, file_size) with
staleness rejected at read time.  Switch to pure path-stem keying: foo.ifc
and foo.ifcdb/ both resolve to foo.ifcview, so the same cache serves either
source format.  Staleness is user-managed (delete the sidecar to force a
rebuild), which also lets sidecars be copied or moved independently of the
source.

v8 header drops the source_file_size field.

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

src/ifcviewer/SceneLoader.h

@@ -63,7 +63,6 @@ public:
 
     QString filePath(uint32_t mid) const;
     QString displayName(uint32_t mid) const;
-    uint64_t fileSize(uint32_t mid) const;
     ifcopenshell::file* ifcFile(uint32_t mid) const;
 
 signals: