Port to Craft 6 (native, adapter-free) via craft-6-plugin-porting skill

[lukeholder]

Summary

Ports Element API 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\ElementApi namespace with Laravel-style folders (Http/Controllers, Events, Contracts, Resources, Transformers, Serializers, Support).
• Native Plugin base class and PluginSettings. The endpoints in config/craft/element-api.php are registered as Laravel routes at boot, using Laravel route syntax (news/{entryId}.json). Endpoint functions can type-hint Illuminate\Http\Request, and int-hinted parameters only match digits.
• DefaultController is now an invokable ElementApiController. Response caching uses ElementCaches and the dependency-aware cache (still tagged element-api, and invalidated when an element is saved). cache: true uses the cacheDuration setting as its TTL, as before.
• Response headers are set with ResponseHeaders::add(). API responses no longer set cookies.
• DataEvent and EVENT_BEFORE_SEND_DATA are replaced by the SendingData Laravel event (clean break, no compatibility layer).
• Registers the "Element API responses" option in Utilities → Clear Caches.
• Removes the generateTransformsBeforePageLoad override, the pathParam guard and the Yii PHPStan stub.
• Adds UPGRADE.md and updates the README and CHANGELOG.

Testing

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

• list, paginated, single (route param), cached, JSON Feed and JSONP endpoints return byte-identical bodies and headers to the same endpoints running on the adapter (Stage 1 of the port)
• OPTIONS preflight with a CORS header set from an endpoint function, 404 and 500 JSON error responses, no Set-Cookie, X-Robots-Tag: none
• the response cache is stored, expires after its TTL, and is invalidated when an entry is saved
• plain, pretty and JSONP encoding compared with yii\helpers\Json on <, &, quotes, slashes and unicode
• paginate: false, contentType, pretty, resourceKey, meta, pageParam, and the dataArray and jsonApi serializers
• Request injection into an endpoint function, the deprecated first setting, an invalid elementType
• the SendingData event, the Clear Caches option, a request from a logged-in admin
• install, uninstall and reinstall

Not verified / follow-ups

• PHPStan and ECS were not run. ecs.php, phpstan.neon, the dev dependencies, composer.lock and .github/workflows/ci.yml still target Craft 4/5.
• Not exercised: a custom transformer class using includes and excludes (the README example), a custom class resource, the live-preview cache bypass, and serving requests from a route cache (route:cache itself ran fine).
• As of Craft 6.0.0-alpha.20, categories, tags and global sets have no native Craft 6 classes (they only exist in the adapter), so they can't be queried without it yet. The README says so.
• php artisan config:cache fails in a project that uses endpoint functions, because closures can't be serialized (documented in the README and UPGRADE.md).
• Only a single site was tested, and Craft 6 is still in alpha.
• This repo has no 5.x branch, so the PR targets 4.x.

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