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-full/MainWindow.cpp

@@ -470,8 +470,6 @@ void MainWindow::setupLoader() {
     session_state_->createLoader(viewport_widget_->viewport());
     viewport_view_ = new modules::viewport::ViewportView(
         session_state_, viewport_widget_->viewport(), this);
-    modules::models::commands::addHandlers(
-        *session_state_, *viewport_widget_->viewport(), *this);
 
     // Load errors surface through SessionState as a session-level signal; the
     // status text + progress are already cleared there, we only show the modal.