ifcviewer-web: export HEAPU8 for heap-size diagnostics

Lets tooling read the wasm heap size (e.g. to verify a large sidecar
streams by byte range instead of loading whole, and to watch memory while
battle-testing big models over the network). Standard emscripten runtime
method, zero size cost.

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

src/ifcviewer-web/CMakeLists.txt

@@ -105,7 +105,10 @@ target_link_options(IfcViewerWeb PRIVATE
     # 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_RUNTIME_METHODS=['ccall']"
+    # 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.
+    "-sEXPORTED_RUNTIME_METHODS=['ccall','HEAPU8']"
     # Streaming + chunked geometry want a heap that can grow as buffers
     # arrive. 256 MB initial, 2 GB ceiling (matches the wasm32 pointer
     # cap; --shared64 / MEMORY64 would lift this later if we need it).