Codex/php84 compat tests

[callinmullaney]

This PR does the following:

• Updates Emulsify Tools to require PHP 8.4+.
• Updates the README compatibility notes to document Drupal 11.3+, PHP 8.4+, and Drush 13+ expectations.
• Adds a GitHub Actions PHP compatibility workflow that runs PHP linting against PHP 8.4 and 8.5.
• Updates ThemeNamespaceRegistry to better protect default Drupal module/theme Twig namespaces while allowing intentional namespace reuse.
• Adds support for components.allow_default_namespace_reuse to opt into reusing an extension’s default namespace.
• Allows namespace reuse when an extension explicitly defines its own matching namespace in components.namespaces.
• Refactors several new Finder() usages to use PHP 8.4-compatible syntax without unnecessary wrapping parentheses.
• Adds unit test coverage for BemTwigExtension.
• Adds unit test coverage for TwigAttributeManager.
• Adds unit test coverage for SubThemeGenerator.
• Adds unit test coverage for namespaced Twig template validation in ThemeNamespaceRegistry.

Related Issue(s)

• N/A

Notes:

• This PR builds on the release-2 branch and assumes the Drupal 11.3+, PHP 8.4+, and Drush 13+ compatibility direction from that release line.
• The PHP compatibility workflow currently runs PHP syntax linting only; it does not run the full PHPUnit test suite.
• The namespace protection changes intentionally preserve default Drupal namespace safety while allowing explicit opt-in reuse through components.allow_default_namespace_reuse.

Functional Testing:

• Confirm Composer accepts the package on PHP 8.4+.
• Confirm Composer rejects or flags incompatible PHP versions below 8.4.
• Run composer install.
• Enable the module in a Drupal 11.3+ site with drush en emulsify_tools -y.
• Rebuild caches with drush cr.
• Confirm the site rebuilds the service container without PHP errors.
• Confirm the GitHub Actions PHP Compatibility workflow runs on the PR.
• Confirm the PHP Compatibility workflow passes for PHP 8.4.
• Confirm the PHP Compatibility workflow passes for PHP 8.5.
• Run PHP linting locally with git ls-files '*.php' | xargs -n 1 php -l.
• Run the available PHPUnit test suite.
• Confirm BemTwigExtensionTest passes.
• Confirm TwigAttributeManagerTest passes.
• Confirm SubThemeGeneratorTest passes.
• Confirm ThemeNamespaceRegistryTest passes.
• Generate a subtheme with drush emulsify "Test Theme".
• Confirm the generated theme appears at themes/custom/test_theme.
• Confirm generated files, directories, and contents are renamed from the starter theme machine name to test_theme.
• Confirm generation fails with a clear exception when the starter source does not contain a *.info.emulsify.yml file.
• Add a custom components.namespaces entry to a test theme and confirm namespaced Twig templates still resolve correctly.
• Attempt to define a namespace that conflicts with an installed module or theme namespace and confirm it is protected unless explicitly allowed.
• Add components.allow_default_namespace_reuse to an extension and confirm its default namespace can be reused.
• Define a matching namespace under components.namespaces and confirm that namespace reuse is allowed.
• Confirm malformed namespaced Twig references still fail validation.
• Confirm supported namespaced template extensions still include .twig, .html, and .svg.
• Confirm unsupported namespaced template extensions, such as .php, are rejected.

Security

Security checks that should be reviewed

• Confirm protected Drupal module/theme Twig namespaces cannot be shadowed unintentionally.
• Confirm namespace reuse is only allowed when explicitly opted into through components.allow_default_namespace_reuse or a matching namespace definition.
• Confirm namespaced template validation continues to reject malformed template names and unsupported file extensions.
• Confirm the subtheme generator only renames and rewrites files inside the generated theme directory.
• Confirm PHP 8.4+ syntax changes do not alter runtime behavior beyond compatibility/readability improvements.

Accessibility

Should this be checked for this feature?

• No dedicated accessibility review is required for the PHP compatibility workflow, Composer constraint, unit tests, or namespace protection changes.
• Spot check rendered templates that rely on theme-defined Twig namespaces to confirm expected markup, classes, IDs, ARIA attributes, and semantic structure are preserved.