Replaces the standalone wgpu-only clear-color spike in main_web.cpp
with a real WebViewportHost implementation: surface creation via the
emdawnwebgpu canvas-selector source, framebufferSize through
emscripten_get_element_css_size + dpr, requestFrame as a deferred
flag the RAF main_loop consumes, quit through emscripten_force_exit.

main_web.cpp now does the same lifecycle the desktop initWgpu shell
does: core_.initWgpu(web_limits=true) → buildPipelines → buildHiz/
Edge/Pick. The render loop runs core_.render() once per RAF tick when
the host has flagged a frame pending, with a surface reconfigure on
size changes.

Builds clean under emcc 6.0 + emdawnwebgpu (1.4 MB wasm, 278 KB JS
glue). Renders an empty scene with the configured background — the
plumbing is end-to-end through the same ViewportCore code path the
desktop build uses. No sidecar load yet: that lands with the
emscripten_fetch streaming backend (#88).

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.