feat: add configurable page performance display

[soulteary]

Summary

• add a disabled-by-default page performance must-use plugin under Settings → Page Performance
• support the exact lowercase WORDPRESS_PAGE_PERFORMANCE_ENABLED=true|false environment override while preserving the stored administrator setting
• show server-side generation time, current PHP memory, and peak PHP memory in the front-end/admin toolbar
• add the same information to the bottom of normal public pages
• reconcile the managed MU Plugin into existing persistent WordPress volumes
• document the public visibility and the difference between PHP generation time and browser/network timing

Behavior

• no output is emitted by default
• a present environment variable makes the setting read-only in WordPress
• an invalid environment value fails closed, keeps output disabled, and shows an administrator notice
• the toolbar receives a late footer-time update so the displayed sample is closer to completed PHP page generation

Validation

• git diff --check
• PHP syntax checks for the plugin and its regression test
• tests/test-sqlite-wordpress-performance.php
• tests/test-mu-plugin-metadata.sh
• tests/test-entrypoint-reconcile.sh
• tests/test-documentation.sh

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[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-01T17:55:24.870407Z	fef0f9e	PR opened

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