Split Admin god-class into focused collaborator classes.

[shadyvb]

Fixes XWPENG-49.

Stacks on #1991 (ticket/XWPENG-48-plugin-boot).

Admin had grown into a god-class (~1,900 lines) mixing menu registration, asset loading, screen rendering, Ajax handlers, and purge/erase scheduling. This PR extracts those concerns into six focused collaborator classes while keeping Admin as a thin façade for cross-cutting behaviour (init, capabilities, notices, shared constants, and plugin action links). Hook registration is colocated on each collaborator using named array( $this, 'method' ) callbacks; runtime behaviour and hook names are unchanged.

Summary

• Admin_Menu — admin menu/submenu registration, screen IDs, and multisite site-access gating.
• Admin_Assets — admin_enqueue_scripts, admin body class, and menu CSS.
• Admin_Screen_Records — records list-table construction and render callback.
• Admin_Screen_Settings — settings page render, option registration, and feature-request notice.
• Admin_Ajax — reset, orphan-meta cleanup, filter autocomplete, and redirect confirmation notices.
• Admin_Purge — TTL auto-purge scheduling (Action Scheduler / WP-Cron), batched erase, orphan-meta reaper, and large-table cron warnings.
• Admin — retains purge-related constants, capability filters, notice pipeline, init() wiring for Network / Live_Update / Export, and exposes public collaborator properties for Network, Export, and Settings call sites.
• Call-site updates — Network, Export, Settings, and Ability_Purge_Records now target collaborators directly (e.g. $admin->menu, $admin->purge) instead of monolithic Admin methods.
• Tests — new PHPUnit coverage per collaborator (Admin_Menu_Test, Admin_Assets_Test, Admin_Ajax_Test, Admin_Purge_Test, Admin_Screen_*_Test); Admin_Test slimmed to façade concerns; testcase.php gains helpers for constructing collaborators in isolation.

Checklist

• Project documentation has been updated to reflect the changes in this pull request, if applicable.
• I have tested the changes in the local development environment (see contributing.md).
• I have added phpunit tests.

Release Changelog

• Internal: Split the Admin god-class into focused collaborator classes to improve testability and maintainability. No user-facing behaviour change.

Release Checklist

• This pull request is to the master branch.
• Release version follows semantic versioning. Does it include breaking changes?
• Update changelog in readme.txt.
• Bump version in stream.php.
• Bump Stable tag in readme.txt.
• Bump version in classes/class-plugin.php.
• Draft a release on GitHub.

Test plan

• CI Lint and Test (PHP 8.2 / 8.3 / 8.4) pass
• CI E2E (PHP 8.2 / 8.3 / 8.4) pass
• composer lint-php passes on PR-changed files
• New collaborator PHPUnit suites cover menu, assets, Ajax, purge, and screen rendering
• Existing Admin_Cron_Purge_Test and Scheduler_Handoff_Test updated for collaborator entry points