Issue #3614258: Add CHANGELOG entry for 3.0.0

Adds a CHANGELOG.txt entry to 3.0.x covering the three fixes that have landed since 3.0.0-beta2 and are currently unreleased:

• #3459820 — "Call to protected method t()", the ContextAwarePluginAssignmentTrait / StringTranslationTrait collision, resolved with an explicit insteadof. Landed as f4e7615, credit lawxen.
• #3531242 — the missing : void return type on submitConfigurationForm(). Drupal 11.2 declared that return type on BlockBase, which made the untyped override a fatal at class load and stopped drush updatedb outright. Core reverted it in 11.3, so the fatal is specific to 11.2.x. Landed as 039cb78, credit Seth Hill.
• #3614068 — the orphaned image component configuration. Landed as a2ff2bf via !14 (merged).

It also records three things that are not the subject of any issue of their own, so the changelog is the only place a user would learn about them. Two were found while fixing #3614068:

• layout_builder_kit.image_component had no schema definition at all on this branch, so the settings form was writing a config object that nothing described.
• layout_builder_kit.settings was declared type: config_entity when it is simple config.

And separately: the test suite had never actually executed — both test files carried a plural Tests suffix, which PHPUnit's *Test.php discovery does not collect, so the suite silently ran nothing.

Version choice

The entry is written as 3.0.0 — a stable release. This is the maintainer's deliberate choice: rather than continuing the beta track with another beta3, 3.0.x is being promoted to stable. The entry was originally drafted as 3.0.0-beta3 and has been amended.

One consequence worth stating explicitly: Drupal's security advisory policy only covers projects with a stable release. Every layout_builder_kit release to date has been alpha or beta, so the project currently sits outside that policy. Tagging 3.0.0 stable brings 3.0.x under security team coverage, which changes how any future security-relevant bug must be handled — reported privately through the security advisory process rather than in the public issue queue.

Notes

• Documentation only. No code changes, no behaviour change.
• This supersedes !14 (merged)'s sibling MR !15 (closed), which was opened from a branch in the main project and therefore never appeared on the issue queue. It has been closed.
• Merges on this project fast-forward, so the Closes #3614258 keyword will not auto-fire — the issue will need to be marked Fixed manually.
• This branch has no GitLab CI, so no pipeline will run here — see #3614091.