[lukeholder]

Summary

Ports Guest Entries to Craft 6 as a fully native plugin. It does not require craftcms/yii2-adapter.

• Requires craftcms/cms ^6.0.0 and PHP ^8.5.
• Moves to the CraftCms\GuestEntries namespace with Laravel-style folders (Http/Controllers, Events, Data).
• Native Plugin base class, PluginSettings with a property hook, and settingsForm() replacing the Twig settings template.
• SaveController is now an invokable class routed from routes/actions.php, so actions/guest-entries/save is unchanged.
• Yii events and SaveEvent are replaced by SavingGuestEntry, SavedGuestEntry and GuestEntrySaveFailed (clean break, no compatibility layer).
• Removes the two legacy settings migrations. The plugin has no tables.
• Adds UPGRADE.md and updates the README for the Craft 6 form idioms (old(), errors, session()).

Testing

Tested in a Craft 6.0.0-alpha.20 host with no adapter and no yiisoft/yii2:

• install, uninstall and reinstall
• settings page render and save
• front-end form round trip (failed post, successful post, redirect object templates)
• Ajax responses (200, 400)
• CSRF on (419 without a token) and off (200)
• events on the normal, rejected, spam and failed paths, using throwaway listeners

Not verified / follow-ups

• PHPStan was not run. ecs.php, phpstan.neon, the dev dependencies and .github/workflows/ci.yml still target Craft 4/5.
• Custom-field error keys (for example errors.get('fields.body')) are not tested.
• A settings save that fails validation was not re-checked (a known alpha TypeError in Craft).
• The 5.x branch already contains a separate Craft 6 port and conflicts with this branch, so this PR targets 4.x.

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