ifcviewer-web: load many models from the URL + per-model loading panel

Query string: auto-load a whole federation, not one model. Accepts repeated
params (?model=a&model=b&…) and/or a comma list (?models=a,b,c); each becomes
its own streamed byte-source, clearing the embedded sample once then streaming
concurrently into one scene. A failed URL is reported without aborting the rest.

Loading UI: the bare aggregate chunk count didn't reveal the federation state,
so surface per-model progress. ViewportCore::streamingModelCount() +
streamingModelProgress(idx,…) (ordered by model_id = load order) feed
main_web's ifcv_model_count_c / ifcv_model_{resident,total}_c. shell.html draws
a panel: "Loading N models — X done · Y streaming · Z waiting · N MB" plus one
segment per model (blue fill while streaming, green when resident, grey while
its metadata is still pending) — so parallel loading and how many remain are
visible at a glance.

Verified: 10 models from a 10x ?model= query stream in parallel; the panel
steps 10 waiting → streaming → all done. 111/111 unit + 6/6 web smoke pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ifcviewer/ViewportCore.h

@@ -395,6 +395,14 @@ public:
     // (still in the metadata phase). Cheap; safe to poll every frame.
     void streamingProgress(int& resident_chunks, int& total_chunks) const;
 
+    // Per-model progress for a federation loading UI. count() is how many
+    // models have metadata (are in the scene); progress(idx,…) gives the
+    // idx-th model's resident/total chunks, ordered by model_id (= load order)
+    // so each model keeps a stable UI slot as it streams.
+    int  streamingModelCount() const;
+    void streamingModelProgress(int idx, int& resident_chunks,
+                                int& total_chunks) const;
+
     // Direct-load (bonsai-side) entry points. Bonsai's SceneLoader feeds
     // the viewer one mesh + one instance at a time, then calls
     // finalizeModel once everything's staged. The staging map lives on