Add fixtures and tests for DesignGuard::missingStylesheets()

Plan step 41(ii): exercises the tag-to-stylesheet mapping (plain, prefixed and
namespaced spellings), a package stylesheet's own imports counting toward its
dependents (split-button.css satisfies button.css and menu.css too),
->links() needing pagination.css, the always-required foundation.css, and a
package tag shadowed by the application's own anonymous or class component.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

tests/Fixtures/design-guard/stylesheets/all.css

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+@import '../../../../resources/css/all.css';

tests/Fixtures/design-guard/stylesheets/foundation-only.css

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+@import '../../../../resources/css/foundation.css';

tests/Fixtures/design-guard/stylesheets/missing-foundation.css

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+@import '../../../../resources/css/components/button.css';

tests/Fixtures/design-guard/stylesheets/split-button.css

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+@import '../../../../resources/css/foundation.css';
+@import '../../../../resources/css/components/split-button.css';

tests/Fixtures/design-guard/stylesheets/views/dependencies.blade.php

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+<x-button label="Save" />
+<x-menu></x-menu>
+<x-split-button label="Save" />

tests/Fixtures/design-guard/stylesheets/views/namespaced.blade.php

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+<x-livewire-material::button label="Save" />

tests/Fixtures/design-guard/stylesheets/views/pagination.blade.php

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+{{ $items->links() }}

tests/Fixtures/design-guard/stylesheets/views/plain.blade.php

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+<div>Nothing package-specific here.</div>

tests/Fixtures/design-guard/stylesheets/views/prefixed.blade.php

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+<x-m::button label="Save" />

tests/Fixtures/design-guard/stylesheets/views/shadow-class.blade.php

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+<x-card title="Report"></x-card>

tests/Fixtures/design-guard/stylesheets/views/shadow.blade.php

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+<x-button label="Save" />

tests/Fixtures/design-guard/stylesheets/views/split-button.blade.php

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+<x-split-button label="Save" />