Extract Alerts admin UI and trigger engine into collaborators.

[shadyvb]

Fixes XWPENG-52.

Extracts two collaborators from the monolithic Alerts class so admin UI concerns and record-matching logic can be unit-tested without booting WordPress. Alerts remains the registry for alert types, triggers, and post-type registration; it wires Alerts_Admin_UI and Alerts_Trigger_Engine as public properties so in-plugin callers can reach them directly.

Alerts_Admin_UI owns the Alerts admin experience: submenu registration, network-admin menu URL rewriting, script enqueue on the alerts list screen, meta-box/AJAX form rendering (display_*_box, load_alerts_settings, save_new_alert, get_actions, get_new_alert_triggers_notifications), and alert-specific action links.

Alerts_Trigger_Engine owns record matching: hooks wp_stream_record_inserted, hydrates enabled alerts, and exposes matching_alerts() so filtering logic (including the wp_stream_alert_trigger_check filter via Alert::check_record()) is testable in isolation before send_alert() runs.

Alerts_List quick-edit now delegates display callbacks to $plugin->alerts->admin_ui. Integration tests in Alerts_Test were updated to call the collaborators; new Brain Monkey unit suites cover notification-value mapping, form output, menu URL rewriting, script enqueue guards, matching_alerts() filtering, and check_records() hydration/send behavior.

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

N/A — internal refactor with no user-facing behavior 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.