New figcraft-render-gpu crate (L4): the scene graph as a vello scene on
wgpu 30 (the version eframe 0.36 uses), kept per top-level layer, culled
to the view, read back to the canvas texture. Off by default: the
gpu-canvas cargo feature plus Preferences > GPU canvas or ui.set
{gpuCanvas}. Effects, masks, pattern fills and outline views fall back
to the CPU renderer per frame (ui.inspect canvas.gpuFallback). Exports,
CLI and MCP stay on the CPU. Parity test against the CPU renderer
(ignored: needs a GPU) and a CPU vs GPU frame-time bench example.
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