Adopt the MIT license

[jacklv-coder]

What changed

• relicense ipcheck from Apache-2.0 to the permissive MIT License
• update package metadata, contribution terms, English/Chinese docs, and release version to v0.6.1
• document that English and Simplified Chinese are supported and every unsupported system language falls back to English
• add deterministic locale tests for generic ja_JP and macOS AppleLanguages=ja-JP, plus AppleLanguages=zh-Hans

Validation

• standard MIT text checked against SPDX
• repository history shows a single copyright-owner email identity
• npm test
• bash -n bin/ipcheck test/smoke.sh
• git diff --check
• final Codex CR: no P1/P2 (GPT-5.6 Sol fallback; Luna unavailable in this environment)