ifcviewer-web: streaming loading bar (metadata → geometry progress)

Users had no feedback during the seconds before/while a model streams. Add a
top progress strip + caption driven by the streaming state:
- "Loading model… N MB" while the critical metadata downloads (no chunks yet),
- "Loading geometry — R / T chunks · N MB" as chunks go resident,
- "Loaded — T chunks · N MB", then it hides.

ViewportCore::streamingProgress(resident, total) sums chunk residency across
models; main_web exports ifcv_chunks_resident_c / ifcv_chunks_total_c for
shell.html to poll each RAF. The EM_JS range reader accumulates
Module.__ifcvBytesLoaded so the caption can show MB downloaded. Shown only for
streamed loads (?model= URL / picked file), not the embedded sample.

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']"
+    "-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']"
     # 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.