Document why engines.node requires ≥22.19

[Blackman99]

Why

engines.node requires ≥22.19, but README/docs do not explain why. That looks arbitrary and raises install drop-off.

Ask

Document in README EN/ZH (and/or install docs):

1. What breaks below 22.19 (APIs / runtime features used)
2. Recommended install path (nvm / official / distro)
3. Pointer from the one-shot install script error path

Acceptance

• Both language READMEs (or a shared install doc linked from both) state the rationale
• No hand-wavy "just upgrade" without a concrete reason

Deduped: not covered by #62/#63/#64/#76–#83.