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-full/MainWindow.h

@@ -67,6 +67,7 @@ private slots:
     void onModelTransformations();
     void onObjectPicked(uint32_t object_id);
     void onTreeSelectionChanged();
+    void onTreeContextMenu(const QPoint& pos);
 
     void onLoadStarted(uint32_t mid, QString display_name);
     void onLoadProgressChanged(int percent);
@@ -99,6 +100,15 @@ private:
                              const std::string& type);
     void writeSidecarForModel(uint32_t mid);
     void removeModelUi(uint32_t mid);
+    void removeModel(uint32_t mid);
+    // Returns the model_id whose tree root is `item`, or 0 if `item` is not
+    // a model root (i.e. an element row, or null).
+    uint32_t modelIdForRoot(QTreeWidgetItem* item) const;
+    // Push the federation's `visible` flag for `mid` onto the viewport.
+    // No-op if `mid` is not in the federation map.  Idempotent — safe to
+    // call before the model is finalised on the viewport (hideModel is a
+    // lookup-and-set on models_gpu_; missing entries are skipped).
+    void applyModelVisibilityToViewport(uint32_t mid);
     void applyPendingBenchmark();
 
     // Push a model's CoordinateOperation matrix to the viewport (or