refactor!: modernize Article, Category and StructureElement

[gharlan]

Summary

Applies the same modernization pattern as the language (#6521) and mediapool (#6516) PRs to the structure-element classes.

StructureElement (abstract base)

• typed promoted readonly properties (camelCase: parentId → removed from base, clangId, createDate, …)
• additionalData array carries the remaining cache fields (meta-info etc.)
• new require(int $id, ?int $clang = null): static next to get(), analogous to Language::require()
• abstract fromCache(array): ?static factory dispatched from StructureElement::get
• $path is now list<int> (mirroring MediaCategory); getPath() / getPathAsArray() removed, getValue('path') still returns the pipe-string for BC
• metaInfoPrefix is an abstract get-only property on the base (PHP 8.4 syntax)
• removed StructureElement::$classVars / getClassVars / resetClassVars (only existed to support the old dynamic-property design); the one caller (ArticleHandler::article2startarticle) uses the SQL field-name list directly
• removed dead last_update_stamp cache column

Article (final)

• own private __construct(array $data) parses the cache row, calls parent::__construct(...) once with named args, then sets its own readonly properties — no parent-arg duplication, single-phase init
• categoryId (nullable), templateKey, startArticle as own properties
• no longer carries parentId (semantically didn't make sense — see comment thread)
• getCategory() exposes the containing/own category; getParent() is protected and used internally for the tree walk
• isSiteStartArticle(), isNotFoundArticle(), isStartArticle() live here (article concepts)
• hasTemplate() removed — check null !== $article->templateKey instead

Category (final)

• own private __construct(array $data) like Article
• parentId (nullable for root) sits here
• getParent() is public (visibility widened from protected base)
• fromCache() returns null for non-start-article cache rows (former Category::class === static::class guard inside the base get() is gone)
• categories no longer carry name/priority/template/startarticle fields — those are article-only; cross-type access via getValue returns null

Rector rules

Cover the visible signature changes for downstream addons:

• Article::getCategoryId → categoryId (changed int → ?int)
• Article::getTemplateKey → templateKey
• Article::hasTemplate → templateKey (changed bool → ?string, callers using the bool need manual adjustment)
• StructureElement::getPath → path (changed string → list<int>)
• StructureElement::getPathAsArray → path
• StructureElement getters → properties (getId, getParentId, getClangId, getName, getPriority, getCreateDate/getUpdateDate/getCreateUser/getUpdateUser)
• Category::getName/getPriority → name/priority (both classes have name/priority properties — Article from name/priority columns, Category from catname/catpriority)

Other

• StructurePermission::hasCategoryPerm(?int) — accepts null (mirrors the now-nullable Article::$categoryId)
• CategoryTreeRenderer::formatLi() removed — only one caller (ArticleList::listItem) and only ever invoked with Article, dead Category branch eliminated; logic inlined in ArticleList
• tests: ArticleTest extended with getClosest/getClosestValue/isOnlineIncludingParents data-provider tests, mirroring CategoryTest