feat: support Emulsify Drupal 6.x child theme generation workflow

[callinmullaney]

This PR does the following:

• Hardens the Emulsify Tools 1.x Drush child theme generation command for the Emulsify Drupal 6.x workflow.
• Aligns the Drush command service wiring with the command and generator constructors.
• Adds safe failure behavior when the destination child theme already exists.
• Validates that the emulsify parent theme and Whisk source directory are available before copying files.
• Adds a disposable child theme generation smoke script covering drush emulsify_tools:bake my_theme and the drush emulsify my_theme alias guard path.
• Aligns Drupal compatibility metadata with the paired Emulsify Drupal 6.x floor of ^10.3 || ^11.
• Updates README guidance for Emulsify Drupal 6.x child theme generation.

Related Issue(s)

• None.

Notes:

• Scope is intentionally limited to Emulsify Tools 1.x and the Emulsify Drupal 6.x child theme generation workflow.
• This does not add Drupal core Starterkit generation, Emulsify Drupal 7.x behavior, Emulsify Tools 2.x behavior, Vite support, Emulsify Core 4 checks, Drupal 12 support, hook attributes, or favicon tooling.
• The PR title and branch include a feat: semantic-release signal so merging this PR should produce 1.1.0.

Functional Testing:

• php -l src/Drush/Commands/SubThemeCommands.php
• php -l src/SubThemeGenerator.php
• vendor/bin/phpcs --standard=Drupal src/Drush/Commands/SubThemeCommands.php src/SubThemeGenerator.php
• vendor/bin/yaml-lint emulsify_tools.info.yml emulsify_tools.services.yml
• bash -n .github/scripts/generation-smoke.sh
• git diff --check
• Full disposable Drupal smoke run: blocked locally because PHP lacks pdo_sqlite.

Security

Security checks that should be reviewed

• The generator now fails safely when the destination theme path already exists.
• The smoke script disables Composer audit blocking only inside a disposable compatibility fixture.

Accessibility

Should this be checked for this feature?

• Not applicable. This PR does not change rendered UI, frontend behavior, or Twig helper output.

[github-actions]

🎉 This PR is included in version 1.1.0 🎉

The release is available on GitHub release

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