Start 4.0.0

[samdark]

#226

TODO:

• Move changelog entries to 4.0.0.
• Add UPGRADE.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (e34f2d1) to head (56e8306).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##              master      #266   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity       830       830           
===========================================
  Files             89        89           
  Lines           2231      2231           
===========================================
  Hits            2231      2231           

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[Copilot]

Pull request overview

This PR introduces stricter validation for HTML tag IDs by rejecting an empty-string id value, adds a regression test for the new behavior, and documents the change in the changelog.

Changes:

• Add runtime validation in Tag::id() to throw when $id === ''.
• Add a PHPUnit test covering the empty-string ID case.
• Add a changelog entry documenting the behavior change.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/Tag/Base/Tag.php	Adds empty-string guard + exception in Tag::id().
tests/Tag/Base/TagTest.php	Adds a test asserting an exception is thrown for id('').
CHANGELOG.md	Documents the new Tag::id() behavior.

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