ifcviewer-web: fix three web-only init aborts

1) Async-wait spin loops in initWgpu / probeAndCreatePool / pick /
   screenshot finalize all called wgpuInstanceProcessEvents in a tight
   while. wgpu-native drives queued callbacks from there; Dawn-web
   queues the callback for an event-loop tick that never happens
   because wasm doesn't yield back to JS. Page hung at the first
   await (RequestAdapter) and Firefox flagged the tab as slow.

   New `waitTickInstance` helper calls emscripten_sleep(0) on
   Emscripten (Asyncify unwinds wasm, JS resolves WebGPU promises,
   resume) before wgpuInstanceProcessEvents drains completions into
   our callback. Desktop path is unchanged. All five
   `while (!done) ProcessEvents` sites switch to the helper.

2) streaming_thread_.start() inside initWgpu spawns a std::thread.
   On Emscripten that needs -pthread + COOP/COEP headers from the
   hosting page. None of that is wired yet, so the start is gated
   `#if !defined(__EMSCRIPTEN__)`. The sync chunk-load fallback
   already inside driveStreamingLoads carries the load until #88
   replaces it with emscripten_fetch.

3) wgpuSurfacePresent at the end of render() aborted with
   "wgpuSurfacePresent is unsupported (use requestAnimationFrame via
   html5.h instead)" — Dawn-web composites the canvas at the end of
   the RAF tick automatically. Call skipped on Emscripten;
   WebViewportHost::requestFrame is the RAF-tick driver.

Page now loads, brings up wgpu, configures the surface, and renders
one frame with the configured background. The frame log in the
status overlay shows the first-frame startup spike (~1000 fps from
a 1 ms tick); subsequent frames are paint-on-demand, which on the
empty scene means no frames at all — consistent with the desktop
event-driven model.

src/ifcviewer-web/main_web.cpp

@@ -62,9 +62,12 @@ void main_loop(void* user) {
         app->last_h = h;
     }
 
-    // Only render when something has requested a frame — saves battery
-    // on the still-camera case. The initial frame request is armed by
-    // WebViewportHost's ctor so the canvas always paints once at startup.
+    // Only render when something has requested a frame — CPU stays
+    // idle while the scene is static. WebViewportHost's ctor arms one
+    // initial frame request so the canvas always paints at startup;
+    // subsequent paints come from core_.render() rearming itself
+    // (motion settle, streaming in flight, bench mode) and, once
+    // input is wired (#85), from mouse / key events flagging the host.
     if (app->host.consumeFrameRequest()) {
         app->core.render();
     }