Unify sidecar production via SidecarBuilder

Renamed HeadlessSidecarBuilder to SidecarBuilder and reused it for live
loads. SceneLoader now constructs one per stream load, forwards meshReady
/instanceReady chunks alongside the viewport upload, and finalizes +
writes the sidecar at onStreamerFinished — no more GPU readback path
via ViewportWindow::snapshotModel (removed). Same code path now produces
sidecars for both live loads and the .rdbview offline export.

Sidecar use is opt-in per direction via SceneLoader::setShouldReadSidecar
and setShouldWriteSidecar; both default off so embedders that don't want
caching get a pure-streaming loader. ifcviewer-full and ifcviewer-minimal
opt in.

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

src/ifcviewer/ViewportWindow.h

@@ -161,12 +161,6 @@ public:
 
     void resetScene();
 
-    // Snapshot the finalised model into a SidecarData struct for caching.
-    // Vertices + indices are read back from the GPU; meshes/instances come
-    // from the CPU-side vectors.  Leaves `elements` and `string_table` empty
-    // for the caller to fill in.
-    bool snapshotModel(uint32_t model_id, SidecarData& out) const;
-
     // Restore a finalised model from a cached SidecarData struct.  Replaces
     // any existing state for model_id and marks it drawable.
     void applyCachedModel(uint32_t model_id, SidecarData data);