WPCS-modernize the relocated inc/ files and drop their PHPCS exclusions

[donnchawp]

This issue was generated by AI.

Follow-up to #1061 (split wp-cache.php into inc/ files, PR #1065 / ADR-0001).

Background

The split relocated legacy procedural clusters verbatim (pure move, no behaviour change). Each relocated file carries pre-existing WPCS debt — loose comparisons (==/!=), non-strict in_array/array_search, missing escaping, legacy spacing, etc. To keep the move behaviour-preserving and the PR lint-clean, those files are excluded from PHPCS in .phpcs.xml.dist:

• inc/plugins-cookies.php
• inc/cache-files.php
• inc/htaccess.php
• inc/settings-forms.php
• inc/preload.php
• inc/lifecycle.php
• inc/admin-notices.php
• inc/admin-ui.php

(inc/boost.php and newly authored files are not excluded and stay fully linted.)

Goal

Modernize each excluded file to satisfy the repo's .phpcs.xml.dist standard, then remove its exclude-pattern so it is linted again.

Constraints

• Behaviour-preserving. Several flagged rules are not cosmetic — ==→===, non-strict in_array/array_search, and @-silencing changes can alter behaviour. Verify each change against the test net (composer test-php, make test-integration, composer test-e2e) rather than blanket-applying phpcbf.
• Do it one file per PR/commit so each is reviewable and revertable, and so the changed-lines linter stays manageable.
• Note: full-file phpcs on these big procedural files aborts on a pre-existing PHPCompatibility Internal.Exception; run it with php -d error_reporting='E_ALL & ~E_DEPRECATED & ~E_USER_DEPRECATED' to get the sniff list (see how Refactor: split wp-cache.php into per-responsibility inc/ files (pure move, test-net first) #1061 surfaced them).

Out of scope

Global-state reduction / class extraction (that's the deeper follow-up, e.g. turning inc/preload.php into the #1047 item-5 preload state machine). This issue is WPCS cleanup only.