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-web/CMakeLists.txt

@@ -104,7 +104,7 @@ target_link_options(IfcViewerWeb PRIVATE
     # EMSCRIPTEN_KEEPALIVE alone keeps the symbols in the binary but doesn't
     # add them to Module. ccall lets shell.html pass a JS string (the ?model
     # URL) to load_sidecar_from_url_c without manual heap marshalling.
-    "-sEXPORTED_FUNCTIONS=['_main','_raf_tick_c','_load_sidecar_from_blob_c','_load_sidecar_from_url_c','_ifcv_on_range_done','_ifcv_source_ready','_ifcv_chunks_resident_c','_ifcv_chunks_total_c']"
+    "-sEXPORTED_FUNCTIONS=['_main','_raf_tick_c','_load_sidecar_from_source_c','_clear_scene_c','_ifcv_on_range_done','_ifcv_chunks_resident_c','_ifcv_chunks_total_c']"
     # ccall: shell.html passes the ?model URL string to load_sidecar_from_url_c.
     # HEAPU8: lets tooling/tests read the wasm heap size (e.g. to verify a large
     # sidecar streams by range instead of loading whole). Standard, zero-cost.