Run WebGPU browser tests in CI

[ibgreen]

Summary

• enable Chromium's software WebGPU adapter for the headless browser job
• preserve the GitHub Actions Chrome channel while applying software-renderer flags
• explicitly select Dawn's SwiftShader WebGPU adapter and initialize the matching GPU path
• serialize browser files only in software-WebGPU mode
• avoid explicitly destroying shared Linux SwiftShader WebGPU devices between browser files; WebGL devices are still destroyed explicitly
• give the broad portable-layer rendering cases additional timeout headroom
• document WebGPU CI coverage in docs/whats-new.md

This draft is intentionally independent of #705 and targets master directly. The WebGPU layer-port PR does not depend on this branch and can land separately.

Current status

Local software-WebGPU validation passes, but GitHub's Ubuntu runner still fails the headless job: the first native WebGPU rendering file loses Dawn's external instance, followed by six failures across five WebGPU browser files.

OperationError: A valid external Instance reference no longer exists.

The failure persists with browser files serialized, explicit WebGPU-device teardown removed, and Chromium's documented --use-webgpu-adapter=swiftshader / --use-gpu-in-tests configuration. This draft therefore remains an isolated CI investigation; it is not ready to land and does not block #705.

Chromium reference: webgpu-swiftshader flag configuration.

Likely next steps are to capture chrome://gpu and adapter diagnostics on the runner, then determine whether the hosted Chrome/Dawn build needs a different Linux image, Vulkan setup, or dedicated GPU-capable runner.

Validation

• yarn lint
• yarn test: 168 files passed; 1,420 tests passed, 9 skipped
• local GITHUB_ACTIONS=true DECK_GL_COMMUNITY_SOFTWARE_WEBGPU=true yarn test-headless: 56 files passed; 438 tests passed, 0 skipped
• GitHub Actions test-headless: 432 passed, 6 failed across five WebGPU files

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Coverage

Stmts: 51.01% ▼-0.01% | Branch: 42.11% | Funcs: 49.15% | Lines: 51.12% ▼-0.01%

Test Files 168 passed (168)