test: run the WebGPU canvas presentation tests on macOS only

[Ernxst]

Harness: Claude Code
Agent: none
Model: anthropic/claude-opus-5

Task statements

1. Run CI on Linux and Windows for the startup fix in fix: stop building HTTP clients before the first frame #593, then merge it. (Working-directory and workflow instructions omitted.)

The two WebGPU canvas presentation tests now run on macOS only. Test GPU canvas presentation exists only in the macOS test-support build, so on Windows both failed in CI run 35665838665:

FAIL canvas.test.tsx > retained canvas element > clears successive native WebGPU canvas frames and locks the context type
Error: Test GPU canvas presentation requires macOS test-support
FAIL native-webgpu-canvas.test.tsx > native WebGPU canvas presentation > composites, clips, scrolls, replaces, and retires two retained GPU canvas textures
Error: Test GPU canvas presentation requires macOS test-support

They landed while CI was switched off. On macOS both still run and pass.

Risk: none outside the two test files.

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