Complete emsdk/WebGPU toolchain hardening foundation

[ford442]

Summary

Most of the toolchain + device-policy foundation from the 2026-07-22 audit landed in #457. This PR closes the remaining gap: TETRIS_CPP_WEBGPU=auto no longer attempts the removed legacy -s USE_WEBGPU=1 flag when .emsdk-version is ≥ 5.0, so a clean npm run cpp:release on the pinned emsdk links emdawnwebgpu without a wasted failed link.

Also adds a buildCanvasConfiguration unit test and aligns cpp/README.md with the actual auto port order on the pinned toolchain.

Acceptance checklist

• .emsdk-version is 5.0.7 (matches CI cache key + cpp/README.md)
• auto prefers emdawnwebgpu; on pinned 5.x, legacy is skipped intentionally (logged)
• build-info.json emits wasmBytes/jsBytes; cpp-renderer.yml CI asserts both
• requestGpuAdapterAndDevice has requiredLimits, centralized buildCanvasConfiguration, GameSettings.gpuPower wiring — covered by tests/gpu-context.test.ts
• "type": "module" in package.json — ESLint runs clean

Changes in this PR

• scripts/build-cpp.mjs: shouldSkipLegacyWebGpu() — when pin major ≥ 5, auto → ['EMDAWN', 'NONE'] with an explanatory log line
• tests/build-cpp.test.ts: assert skip logic + jsBytes contract
• tests/gpu-context.test.ts: buildCanvasConfiguration premultiplied-alpha test
• cpp/README.md: document auto behavior on pinned 5.x

Not in scope (deferred)

• C++ WGPUPresentMode_Mailbox experiment for high-end — still Fifo + premultiplied; can follow once shader parity work resumes
• Replacing emdawn with Rust wgpu (research already decided against)

Test plan

• npm test (213 tests)
• npm run lint (no MODULE_TYPELESS_PACKAGE_JSON warning)
• npm run cpp:release on emsdk 5.0.7 → webGpuBackendId: "emdawn" with no legacy attempt (CI cpp-renderer.yml)

  

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

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Run ID: 4598624f-8324-4a27-8f50-74a777122d06

📥 Commits

Reviewing files that changed from the base of the PR and between 6c8ee77 and f3fa65c.

📒 Files selected for processing (4)

• cpp/README.md
• scripts/build-cpp.mjs
• tests/build-cpp.test.ts
• tests/gpu-context.test.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch cursor/emsdk-webgpu-hardening-623a

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