chore: align PHP coding standards

[rbcorrales]

Summary

1. Adds PHPCompatibilityWP to the existing WordPress and VIP PHPCS ruleset.
2. Limits PHPCS to PHP files and excludes VIP JavaScript sniffs from this PHP plugin ruleset.
3. Adds the missing PHPCS installer and PHPCompatibilityWP Composer dev dependencies.
4. Applies PHPCBF mechanical formatting across the existing plugin PHP files.
5. Updates PHPUnit within the existing 9.x constraint to clear the Composer audit advisory.

Notes

This is a baseline cleanup PR, not full standards compliance. PHPCBF fixed 25,059 violations, but PHPCS still reports semantic follow up work around escaping, nonces, prepared SQL, direct database access, Yoda conditions, translator comments, and docblocks.

Verification

1. ./vendor/bin/phpcbf --standard=phpcs.xml.dist
2. find includes admin -name '*.php' -print0 | xargs -0 -n 1 php -l
3. php -l wpvdb.php
4. git diff --check
5. ./vendor/bin/phpcs --standard=phpcs.xml.dist --report=summary
6. composer audit --format=plain

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