Fix: use home_url() instead of site_url() in get_current_url_supercache_dir

[kraftbj]

Summary

• Replace site_url() with home_url() in get_current_url_supercache_dir(), since permalinks are relative to the home URL, not the site URL (these differ when WordPress is installed in a subdirectory).
• Use wp_parse_url( $permalink, PHP_URL_PATH ) to extract the URI path instead of string-replacing with $wp_cache_home_path, which doesn't work reliably in multisite installations.
• Add strict type checking to in_array() call.

Originally proposed by @stodorovic in #613.

See #613

Test plan

• Verify cache directory resolution on a standard single-site install.
• Verify cache directory resolution on a multisite install.
• Verify cache directory resolution when WordPress is installed in a subdirectory (site URL differs from home URL).
• Confirm get_current_url_supercache_dir() returns the correct path for individual post IDs.

[Copilot]

Pull request overview

Adjusts supercache directory resolution to use the WordPress “home” URL and more robust URL parsing, improving correctness for subdirectory installs and multisite setups (see #613 / #613).

Changes:

• Replaces site_url() with home_url() when deriving the permalink-relative cache path.
• Uses wp_parse_url( $permalink, PHP_URL_PATH ) instead of string replacement against $wp_cache_home_path.
• Enables strict comparison for the in_array() check against $cached_direct_pages.

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