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/SceneLoader.h

@@ -60,8 +60,15 @@ public:
     std::vector<uint32_t> addFiles(const QStringList& paths);
     void cancelCurrentLoad();
     bool isLoading() const { return loading_model_id_ != 0 || !load_queue_.empty(); }
+    bool isLoadingModel(uint32_t mid) const { return loading_model_id_ == mid; }
     size_t modelCount() const { return models_.size(); }
 
+    // Drop the loader's tracking for `mid` — its streamer, file path, georef
+    // cache, and queue slot if still pending.  Caller is responsible for the
+    // viewport / UI cleanup; this only releases the loader's own state.
+    // Refuses while the model is the active load (use cancelCurrentLoad first).
+    void removeModel(uint32_t mid);
+
     QString filePath(uint32_t mid) const;
     QString displayName(uint32_t mid) const;
     ifcopenshell::file* ifcFile(uint32_t mid) const;