test: enable WebGPU rendering in headless Chromium - #81

On Windows, Dawn's D3D12 backend fails to create a device because the
bundled Chromium ships a dxil.dll it cannot load in the sandboxed GPU
process (EnsureDXCLibraries -> "DynamicLib.Open: dxil.dll Windows Error:
87"). The adapter is obtained fine; only requestDevice fails. Disabling
the use_dxc Dawn feature falls back to the FXC shader compiler, which
needs no external DLL. macOS (metal) and Linux (vulkan) do not use DXC,
so the flag is applied only on win32.

Pin the cone view to a fixed ref ("cone_view") so the test can address it
regardless of the process-global _vtklocalview_N counter, and assert the
active render window class (webgl vs webgpu) against it.

Run that assertion after every screenshot: getVtkObject() performs a
client-side serialize that currently corrupts the WebGPU render window
permanently (a VTK bug; harmless on webgl), which would blank all later
frames if done mid-sequence. The className comes from static server state,
so ordering it last does not weaken the check.

Fixed asyncio.sleep(0.1) waits were both flaky and masking a real bug: on
(re)mount the canvas starts at its 300x150 HTML default and only reaches
the container size after the 100ms debounced ResizeObserver calls
setSizeAsync, which does not bump the `updated` counter. A short sleep
could capture the wrong-sized frame. WebGPU also presents the updated
frame a frame later than the `updated` event, so an immediate capture
grabs a blank pre-present buffer.

Add Utils.wait_for_render, which polls on requestAnimationFrame until
every canvas drawing buffer matches its layout size (the same
floor(size * dpr + 0.5) formula the component uses) and stays stable for
two frames, giving the compositor time to present. Use it in the cone,
volume, and multi-view tests in place of every sleep.

Add vtk 9.7.20260712.dev0 to the dev group, tests need it to pass.

That dev wheel is published for Windows only as cp313/cp314 (no cp310
win_amd64), so bump the pytest matrix to Python 3.13 and pin uv to the
matrix interpreter via UV_PYTHON, which the workflow previously left to
uv's default.