Add Update URI protection to generated themes

[thetwopct]

Summary

• add Update URI: false to every newly generated clone, blank theme, and child theme
• preserve an existing Update URI when CBT updates metadata for an existing theme
• add regression coverage for generated and existing theme stylesheets

This prevents a custom theme created with CBT from being replaced by an unrelated WordPress.org theme when their slugs collide.

Closes #875.

Testing

• ./vendor/bin/phpcs --standard=phpcs.xml.dist
• targeted PHPUnit suite: 4 tests, 5 assertions passed
• isolated API and font tests implicated by the full-suite run: 6 tests, 24 assertions passed
• JavaScript unit tests: 15 tests passed
• JavaScript, CSS, Markdown, and package JSON linters passed
• production build passed with existing dependency, Sass, icon, and bundle-size warnings

The complete PHP suite currently reports six order-dependent failures in existing API and font tests; all six pass when run in isolation. The new stylesheet test suite passes both independently and as part of the complete run.