ifcviewer-web: load user sidecars via a file-browse button (#46)

Adds an "Open .ifcview…" button that opens the browser's native file
chooser. The picked file's bytes are written into MEMFS and a new
exported entry point, load_uploaded_model_c, reads them back via the
existing loadSidecarFromPath: it resetScene()s the current model
(replace, not append), loads the sidecar, and viewAll()s it. Geometry
becomes resident over subsequent frames through render()'s inline
driveStreamingLoads, same as the embedded sample.

Deliberately a file picker, not drag-drop: browser file drag-drop needs
an X11 drag source (a file manager) to drag *from*, which a minimal WM
(ratpoison) doesn't provide. The native chooser is WM-independent.

Exports _load_uploaded_model_c and the FS runtime method; URL/byte-range
fetch of remote models stays for #88.

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

src/ifcviewer-web/CMakeLists.txt

@@ -96,10 +96,17 @@ target_link_options(IfcViewerWeb PRIVATE
     # main() to set noExitRuntime as a side effect — registers a RAF
     # that starves the device promise (observed: ~10s delay in Firefox).
     "-sEXIT_RUNTIME=0"
-    # Expose the C tick entry point to JS so shell.html's RAF loop
-    # can call Module._raf_tick_c. EMSCRIPTEN_KEEPALIVE alone keeps
-    # the symbol in the binary but doesn't add it to Module.
-    "-sEXPORTED_FUNCTIONS=['_main','_raf_tick_c']"
+    # Expose the C entry points to JS. _raf_tick_c drives the RAF loop
+    # (shell.html); _load_uploaded_model_c loads a user-picked sidecar
+    # that JS has written into MEMFS. EMSCRIPTEN_KEEPALIVE alone keeps
+    # the symbols in the binary but doesn't add them to Module.
+    "-sEXPORTED_FUNCTIONS=['_main','_raf_tick_c','_load_uploaded_model_c']"
+    # FS lets shell.html's file-browse handler write the picked file's
+    # bytes into MEMFS before calling _load_uploaded_model_c (which
+    # fopen()s the virtual path). Drag-drop is intentionally not used —
+    # it depends on an X11 drag source (file manager), which a minimal
+    # WM may not provide; the native file chooser is WM-independent.
+    "-sEXPORTED_RUNTIME_METHODS=['FS']"
     # 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).