ifcviewer-full: hide and remove model actions

Right-click a model root in the Elements tree to get Hide/Show and
Remove.  Hide flips the federation's per-model visible flag (already
round-tripped to .ifcfed), pushes ViewportWindow::hideModel/showModel,
and italicises + greys the tree root as a visual cue.  Remove drops
the model from the viewport, the SceneLoader (streamer + caches), the
MainWindow UI maps and tree, and the Federation — disabled while the
model is the active load.

Visibility is reapplied on each model's load completion (sidecar or
stream), so a federation saved with hidden models opens with them
hidden.  clearScene() now also drops SceneLoader state so streamers
no longer leak across federation transitions.

API additions:
- Federation::setModelVisible + modelVisibilityChanged signal
- SceneLoader::removeModel + isLoadingModel

Tests cover the setter (dirty + signal + idempotence + unknown id);
extends the existing round-trip test to actually exercise the
visibility load/save it always claimed to.

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

src/ifcviewer/Federation.h

@@ -210,6 +210,7 @@ public:
     void setConfig(const FederationConfig&);
     void setFederatedFalseOrigin(const FederatedFalseOrigin&);
     void setModelTransformation(const QString& fed_id, const ModelTransformation&);
+    void setModelVisible(const QString& fed_id, bool visible);
 
     // Accessors
     const std::vector<Model>& models() const { return models_; }
@@ -232,6 +233,7 @@ signals:
     void configChanged();
     void federatedFalseOriginChanged();
     void modelTransformationChanged(const QString& fed_id);
+    void modelVisibilityChanged(const QString& fed_id, bool visible);
 
 private:
     void setDirty(bool d);