ci(source-map-validation): run core compat test on parser pull requests

[luojiyin1987]

Summary

Closes #104

The source-map validation workflow now also runs on parser pull requests instead of only on manual dispatch and the weekly cron.

A parser PR that breaks lint-md core now fails before merge, not days later.

Changes

• Add a pull_request trigger to .github/workflows/source-map-validation.yml.
• Limit the trigger to high-risk paths: src/**, etc/**, package.json, scripts/test-package.mjs.
• Keep workflow_dispatch and the weekly cron. Core moves independently; its own dependency changes can still break against an unchanged parser.

Notes

• Docs-only and CI-only PRs do not run this workflow.
• The workflow needs no secrets. It keeps working on fork PRs.
• If this check later becomes a required status check, skipped runs (PRs outside the path filter) will block merge. Keep it non-required, or add a paths-filter gate job at that point.