ifcviewer-web: end-to-end rendering on Chrome + Firefox

Wire up wgpu init + scene load + RAF render so the embedded sample
sidecar paints on the canvas in both browsers.

Root-cause fix: BufferPool::addSubBuffer spin-waited on
PopErrorScope, which resolves via JS microtask on Dawn-web. The
spin blocked the JS event loop, so the microtask never fired and
the first allocation hung the page indefinitely. Skip the
error-scope dance on Emscripten; trust the buffer pointer.

ViewportCore: add initWgpuAsyncWeb (nested-callback adapter→device
chain with AllowSpontaneous mode, no spin) and loadSidecarFromPath
(Qt-free entry point). waitTickInstance becomes a no-op shim on
web; cull-threads / streaming_thread_ / wgpuSurfacePresent gated
off; chunk I/O runs inline.

main_web.cpp: AppState + initWgpuAsyncWeb → buildPipelines (+
HiZ/edge/pick) → loadSidecarFromPath → ready flag → Module._app_ptr
handoff. The RAF loop lives in shell.html (NOT here) because any
RAF helper called from inside Dawn-web's wgpu Promise.then chain
stalls the device callback.

CMakeLists.txt: EXIT_RUNTIME=0 + Module.noExitRuntime=true (shell)
keeps wasm alive past main() so the device promise lands; no
Asyncify; export _raf_tick_c so shell.html's RAF can call it.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ifcviewer-web/CMakeLists.txt

@@ -80,12 +80,26 @@ add_executable(IfcViewerWeb
     WebViewportHost.h
 )
 target_link_libraries(IfcViewerWeb PRIVATE IfcViewerCore)
+
 target_link_options(IfcViewerWeb PRIVATE
-    # Asyncify lets us await wgpu's RequestAdapter/RequestDevice/MapAsync
-    # as if they were synchronous. Adds ~30-50% wasm size and a small
-    # per-await runtime cost; acceptable for the spike, revisit when
-    # we ship.
-    "-sASYNCIFY"
+    # No -sASYNCIFY. The web init path is callback-driven
+    # (initWgpuAsyncWeb chains RequestAdapter → RequestDevice via
+    # WGPU's AllowSpontaneous mode + the JS event loop), so we don't
+    # need to await wgpu calls as if they were sync. Asyncify would
+    # also instrument every function reachable from emscripten_sleep,
+    # adding ~30% to wasm size for no win here.
+    #
+    # EXIT_RUNTIME=0 + Module.noExitRuntime=true (set in shell.html)
+    # keeps wasm alive after main() returns so Dawn-web's
+    # RequestAdapter/RequestDevice promise callbacks land. The
+    # alternative — calling emscripten_set_main_loop_arg early in
+    # 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']"
     # 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).
@@ -96,6 +110,12 @@ target_link_options(IfcViewerWeb PRIVATE
     # sidecar byte-range loader. Not used yet by the scaffold but
     # needed by the upcoming #27 web streaming I/O backend.
     "-sFETCH=1"
+    # Bundle a small sample sidecar into Emscripten's MEMFS so the
+    # scaffold can prove the load path end-to-end without needing
+    # emscripten_fetch + COOP/COEP wiring. The @ separator mounts the
+    # file at the virtual path the wasm uses to fopen() it. Replaced
+    # by an emscripten_fetch + Range backend in #88.
+    "--embed-file=${CMAKE_CURRENT_SOURCE_DIR}/sample.ifcview@/sample.ifcview"
     # Shell template wraps the JS output in our canvas page.
     "--shell-file=${CMAKE_CURRENT_SOURCE_DIR}/shell.html"
 )