Performance: Autoload Plugin Classes

[n7studios]

Summary

Reduces the memory the Plugin uses on each request by loading class files only when they're used, instead of loading every file when the Plugin starts.

• Registers convertkit_autoloader() in wp-convertkit.php. The first time a ConvertKit_, CK_ or WP_ConvertKit class is used, it loads the class's file from a list of Plugin folders, with the folders used most on the frontend checked first. Files with code outside the class (integrations that hook in directly, plus functions.php and cron-functions.php) are still loaded with require_once.
• Registers Kit's own settings sections in ConvertKit_Admin_Settings::register_sections(), instead of each section file hooking into convertkit_admin_settings_register_sections when loaded. Section files are no longer loaded on every request, so they couldn't register themselves. Tab order and the conditions for MCP, Contact Form 7, Forminator and WishList Member are unchanged. The filter still runs, so third-party sections still work.
• Moves the Form Entries set-screen-option filter into the ConvertKit_Admin_Settings constructor, as it must be registered before admin_init.

Frontend peak memory, OPcache off

	Peak	Plugin overhead
No Plugin	34.51MB	–
main	40.40MB	+5.89MB
This PR	37.06MB	+2.54MB (−57%)

Frontend peak memory, OPcache on

	Peak	Plugin overhead
No Plugin	4.48MB	–
main	4.91MB	+0.42MB
This PR	4.90MB	+0.42MB (−2%)

With OPcache on, compiled code lives in shared memory, so the saving per request is small (~9KB). The main gains are on hosts without OPcache, cold caches, WP-CLI and cron. Frontend requests now include 60 plugin files instead of 155.

This won't fix the specific creator's issue in WP-95, given the memory usage is already low, their environment has a strangely low memory limit of 128M, and their other plugins contribute to the issue - but will hopefully prevent it for other creators in the future.

Testing

• AutoloadTest::testAllClassesCanBeAutoloaded: every class in /includes and /admin is autoloaded from its own file. A class in a folder that isn't in the autoloader's list fails this test.
• AutoloadTest::testNonPluginClassesAreIgnored: the autoloader does nothing for classes that don't belong to the Plugin.
• Existing settings, Form Entries, integration and Integration tests pass unchanged.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

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