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-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_source_c','_clear_scene_c','_ifcv_on_range_done','_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','_ifcv_model_count_c','_ifcv_model_resident_c','_ifcv_model_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.