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/ModelGpuData.h

@@ -287,6 +287,11 @@ struct ModelGpuData {
     // so driveStreamingLoads routes this model through the async web path
     // instead of the MEMFS sync read.
     bool        streaming_from_web = false;
+    // Web analog of streaming_file_path: which registered JS byte-source
+    // (Module.__ifcvSources[id] = a picked File or a remote URL) this model's
+    // chunk + deferred reads pull from. Lets several federated models stream
+    // from different files at once, mirroring the desktop per-model path.
+    int         web_source_id = 0;
 
     // v15 deferred property metadata (web, on-demand). The IFC element tree
     // (elements + string_table — names/GUIDs/hierarchy, for UI/picking, never