Fix release hardening findings

[mehul0810]

Summary

• Scope Menu Cache to classic/non-block themes by default, with a filter for hybrid themes that intentionally use wp_nav_menu().
• Preserve multiline resource hint settings through the React settings save path.
• Normalize Assets Manager current-page IDs so legacy string IDs and new integer IDs match consistently.
• Gate the Assets Manager frontend overlay assets behind manage_options.
• Sample and merge cache stats updates to reduce hot-path writes and avoid overwriting concurrent stats.

Validation

• composer lint
• composer test
• composer phpstan
• git diff --check
• vendor/bin/phpcs -s --no-cache on the PR changed-file set

Fixes #82
Fixes #94
Fixes #95
Fixes #96
Fixes #97

[github-actions]

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