Align runtime support after Node 20 EOL

[lan17]

Priority

P1 — High — make the published runtime support contract truthful before broad production adoption.

Current problem

The package still declares 20 || >=22 and the README says Node 20+, while Node 20 reached end of life on March 24, 2026. CI runs the full development/integration suite on Node 24 and then a packed-consumer test on a floating latest Node 20, so it neither tests the bottom of the declared Node 20 range nor reflects the maintained production runtimes.

Current evidence:

• Published engine range
• Current CI runtime split
• Packed ESM/CommonJS/runtime/declaration harness
• Official Node release status

The resolved Vite/Vitest development-tool floor is not itself the consumer-runtime contract; packed artifacts must be tested independently at the declared minimum.

Open decision

Choose deliberately between:

1. Continue supporting the EOL Node 20 line and pin/prove its exact minimum consumer version, accepting the maintenance and security-support implications.
2. Move DialCache to Node 22+ and test packed consumers at an exact minimum Node 22 release while retaining the full suite on Node 24/current LTS.

Recommendation: choose Node 22+. Production applications should not be encouraged onto an EOL runtime, and this removes the stale 20.0-versus-20.19 framing without tying consumer support to dev-tool requirements.

Acceptance criteria

• Confirm the maintained major-version floor.
• Align engines.node and README requirements.
• Pin the packed-consumer job to an exact minimum supported version rather than a floating major.
• Run real packed ESM and CommonJS cache behavior plus declaration checks at that minimum.
• Keep the full unit, integration, build, and declaration suite on Node 24/current LTS.
• Document the compatibility change in the PR and release notes.

Impact

This changes only the advertised/install-time support matrix and CI proof. It does not change cache keys, Redis protocol, runtime APIs, or stored data.