build: add test artifact bootstrap

[BartWaardenburg]

Summary

• add one root command that builds all generated NAPI and WASM test artifacts
• preserve tracked NAPI loaders by building with no-js
• reuse the same command in CI and contributor setup docs
• enforce the bootstrap contract with the workflow policy test

Verification

• workflow policy test passed after red-green implementation
• clean-worktree artifact bootstrap passed
• tracked NAPI loaders remained unchanged
• pnpm check passed
• pnpm test passed
• independent diff review found no issues

[codspeed-hq]

Merging this PR will not alter performance

✅ 188 untouched benchmarks
⏩ 76 skipped benchmarks¹

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_{Comparing codex/test-artifact-bootstrap (1d3eb48) with main (a307917)}

Footnotes

1. 76 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩