[P2][ci] Add Node.js 22 smoke job to CI #32

Summary

package.json declares Node.js engines >= 22 and documents npx usage, but CI runs Bun-only (bun run gate). Node compatibility regressions would ship unnoticed.

Audit ID: Section 2 Med — Wave 2

Fix

Add CI job (e.g. node-smoke in .github/workflows/ci.yml):

1. actions/setup-node with Node 22
2. bun run build (or use published tarball from npm pack)
3. Minimal script: open({ path, fs: nodeFileSystem() }) → transact set → close → reopen → get asserts value

Keep job fast (<2 min); no full gate duplicate on Node unless desired.

Acceptance criteria

• Node 22 job runs on PR and main.
• Smoke test covers path-backed open/write/reopen.
• Document in docs/TOOLCHAIN.md if needed.
• No changeset (CI only).

Verification

Push branch; CI green on Node job.