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/ViewportCore.h

@@ -257,6 +257,19 @@ public:
     bool initWgpu(bool web_limits);
     void shutdown();
 
+#if defined(__EMSCRIPTEN__)
+    // Async-init driver for the web. The spin-wait pattern in initWgpu()
+    // doesn't work on Dawn-web — RequestDevice's callback never fires
+    // when the caller is parked inside an Asyncify spin loop, even with
+    // AllowSpontaneous + emscripten_sleep yields. The fix is to mirror
+    // the original main_web.cpp spike: nested callbacks, no spin.
+    // Fires `on_complete(true)` once instance + adapter + device + queue
+    // + pool + surface_format_ are all in place; on any failure, fires
+    // on_complete(false). Caller is responsible for calling
+    // buildPipelines + buildHiz/Edge/Pick + scene-load after on_complete.
+    void initWgpuAsyncWeb(std::function<void(bool ok)> on_complete);
+#endif
+
     // ---- Chunk residency (#84-n) ------------------------------------------
     //
     // Build the per-chunk WGPUBindGroup over its current pool slices +
@@ -315,6 +328,14 @@ public:
     // freshly-loaded scene frames itself).
     void applyCachedModel(std::uint32_t model_id, StreamingSidecar metadata);
 
+    // Qt-free sidecar load: readSidecarMetadataOnly + applyCachedModel.
+    // Used by the web build (and any other non-Qt embedder) so the
+    // public ViewportWindow::loadSidecar's QString + QFile triage
+    // tilde-expansion doesn't have to be replicated. Returns 0 on
+    // any failure (device not ready, file missing, magic / version
+    // mismatch) and the freshly-assigned model_id on success.
+    std::uint32_t loadSidecarFromPath(const std::string& path);
+
     // Direct-load (bonsai-side) entry points. Bonsai's SceneLoader feeds
     // the viewer one mesh + one instance at a time, then calls
     // finalizeModel once everything's staged. The staging map lives on