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.cpp

@@ -254,6 +254,17 @@ void Federation::setModelTransformation(const QString& fed_id,
     }
 }
 
+void Federation::setModelVisible(const QString& fed_id, bool visible) {
+    for (auto& m : models_) {
+        if (m.id != fed_id) continue;
+        if (m.visible == visible) return;
+        m.visible = visible;
+        setDirty(true);
+        emit modelVisibilityChanged(fed_id, visible);
+        return;
+    }
+}
+
 void Federation::markClean() {
     setDirty(false);
 }