ifcviewer-web: multi-file loading (federation) via a per-model byte-source

The scene core is already multi-model — models_gpu_ is a map, applyCachedModel
APPENDS, and per-model model_id / object_id rebasing / georef+transformation
are how the desktop federates today. The only web-specific gap was the byte
source: web had ONE global source (__ifcvFile/__ifcvUrl) and reset the scene on
every load, so it could show one file at a time. Desktop meanwhile carries a
per-model source (streaming_file_path).

Mirror that on web: give each model its own web_source_id into a JS source
registry (Module.__ifcvSources[id] = a picked File or a sized remote URL).
beginWebChunkLoad, the metadata bootstrap, and the on-demand deferred fetch all
read from the owning model's source, so several files stream concurrently into
one federated scene — reusing all the shared machinery (viewAll, picking, the
GUID fetch) untouched.

- webReadRangesAsync / ifcvReadRangeInto / ifcvSourceSize take a source id.
- loadSidecarMetadataWeb(source_id, …) appends (no resetScene); main_web
  exposes load_sidecar_from_source_c(id) + clear_scene_c().
- URL size resolution moved to JS (shell.html registers + sizes sources via
  HEAD/Range), retiring the C-side ifcvBeginUrlSource / ifcv_source_ready dance.
- shell.html: source registry + "Open" (replace) / "Add" (append) buttons,
  multi-file selection; ?model= registers a URL source then loads.

Verified: two sidecars from two sources stream into one scene, both fully
resident, zero GPU errors. 111/111 unit + 6/6 web smoke pass.

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

src/ifcviewer/ViewportCore.h

@@ -362,14 +362,12 @@ public:
     // are read via byte ranges, the streaming model is built, and it is tagged
     // web-sourced so each chunk's vertex/index ranges are pulled lazily.
     // Asynchronous — returns immediately and frames the model from the JS
-    // completion callback. resetScene() first to replace.
-    //
-    //  - Blob:  the picked File on Module.__ifcvFile (Blob.slice).
-    //  - URL:   a remote sidecar (HTTP Range); resolves total size first, then
-    //           runs the shared bootstrap via the ifcv_source_ready callback.
-    void loadSidecarFromBlobWeb();
-    void loadSidecarFromUrlWeb(std::string url);
-    void loadSidecarMetadataWeb(std::string source_label);
+    // completion callback. APPENDS the model (federation); call resetScene()
+    // first to replace. `source_id` indexes the JS byte-source registry
+    // (Module.__ifcvSources[source_id] — a picked File or remote URL, already
+    // sized by shell.html); each federated model streams from its own source.
+    // `source_label` is a log/identity tag.
+    void loadSidecarMetadataWeb(int source_id, std::string source_label);
 
     // On-demand fetch of the v15 deferred property block (element tree + string
     // table) for a web-streamed model — what a UI (object tree / selected-name