RFC: define the Node runtime policy before 1.0 #206

Current mismatch

The package currently declares node >=18, while both CI workflows pin Node 20.

As of the 1.0 release window:

• Node's official release table lists v18 and v20 as EOL; v22 and v24 are the maintained LTS lines: https://nodejs.org/en/about/previous-releases
• GitHub runner images removed Node 20 from the toolcache and retain Node 22/24; actions/setup-node can still download 20 on demand, but it is no longer a maintained runner baseline: [All OS] Default Node.js version will be changed from 20 to 22 and Node.js 20 will be removed from runner images actions/runner-images#14029

That leaves three different implied contracts: package metadata says 18+, CI proves only 20, and the platform's maintained baseline is 22/24. A 1.0 support promise should make these agree.

Proposed policy

Recommended:

1. Raise engines.node to >=22 before 1.0.0-rc.
2. Test Node 22 (minimum) and Node 24 (current LTS) on the x64 Windows lane.
3. Run the ARM64 lane on Node 24, unless the full cross product remains comfortably within CI time.
4. Run package smoke on both the minimum and current LTS so install/build behavior is part of the contract.
5. Document the runtime policy and treat dropping a maintained major after 1.0 as a semver-major change.

If preserving Node 18/20 is a real user requirement, the alternative should be explicit: name the support end date and add those versions to CI rather than leaving them in metadata without evidence.

Definition of done

• package.json, README, CI, and package smoke name the same minimum runtime
• maintained minimum/current LTS lanes are green on x64
• ARM64 is green on the chosen LTS lane
• release checklist verifies the declared engine range before publish

This is independent of U-6 (#197); that PR establishes the architecture lane and should not absorb a runtime-policy decision.