fix: align supported Node engine range

[poketopa]

Summary

• require Node.js 20 or later in package.json, matching the oldest runtime covered by binding CI
• update the English and Korean installation guides to state the same support floor

Rationale

PR #209 removed Node.js 18 from the test matrix and the v0.7.0 release notes record that support change. The binding matrix now verifies Node.js 20, 24, and 26 across macOS, Windows, and Linux, while the package metadata and installation guides still advertised Node.js 10-era support.

Validation

• yarn install --immutable
• just format
• just lint
• just typecheck
• just test
• git diff --check

[vercel]

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