refactor!: register addon backend pages via Addon::getPages()

[gharlan]

Summary

Replaces the package.yml page/pages configuration with an overridable Addon::getPages() method that returns Page objects — the last functional bit of package.yml besides default_config.

public function getPages(): iterable
{
    yield new MainPage('addons', $this->name, I18n::msg('my_title'))
        ->setIcon('rex-icon rex-icon-myaddon')
        ->addSubpage(new Page('settings', I18n::msg('settings')));
}

Details

• Path-by-convention preserved: a page without an explicit path falls back to pages/<key>.php, or pages/index.php for the main page whose key equals the addon name. Subpaths keep the dotted-prefix scheme.
• live_mode filtering is now explicit PHP (if (Core::isLiveMode()) return;) instead of a YAML flag.
• Cross-tree insertion: the deferred foo/bar key-matching mechanism is dropped. getPages() runs in load order after the core pages, so Controller::getPageObject() can attach subpages to core/earlier-loading addons; later-loading targets use LoadOrder::Late or the PAGES_PREPARED extension point.
• Top-level pages are typically MainPage (shown in navigation); a plain Page remains valid for hidden, key/URL-reachable entry points (same as core's profile/credits).
• The debug addon is converted and its package.yml removed; the package.yml schema is reduced to default_config.

Net −314 lines. The array-parsing helpers (pageCreate, pageAddProperties) are gone.

Compatibility

Breaking change for addons that defined pages in package.yml — they move to Addon::getPages(). Acceptable on 6.x (no stable release yet).