fix: support markdown-it 15

[peaceroad]

Summary

• replace the removed md.utils.assign helper with Object.assign
• update the development markdown-it version to 15.0.1
• update CI to Node.js 22 because markdown-it 15's dependency tree includes entities@8, which requires Node.js 20.19 or later
• preserve the existing option merge order and behavior

md.utils.assign is no longer available in markdown-it 15, causing plugin registration to fail with TypeError: md.utils.assign is not a function.

The CI update does not change the package's declared runtime support.

Validation

• existing fixture suite passes with markdown-it 15.0.1
• coverage remains at 100%
• lint passes
• ESM plugin registration passes
• generated CommonJS plugin registration passes
• Rollup build passes
• the markdown-it instance is not patched to restore the removed helper

Closes #82.

This pull request was prepared with assistance from GitHub Copilot (GPT).