chore: map npm package links to npmx.dev

[Copilot]

Update README badge links from npmjs.org to npmx.dev.

• [npm-url] and [download-url] now point to npmx.dev/package/compressing
• Registry URL in code example left unchanged (actual API endpoint)

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.70%. Comparing base (15d24cd) to head (6b49dff).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #134   +/-   ##
=======================================
  Coverage   97.70%   97.70%           
=======================================
  Files          19       19           
  Lines        1087     1087           
  Branches      285      285           
=======================================
  Hits         1062     1062           
  Misses         25       25           

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