feat: require the pollora/assets package

[ogorzalka]

Step 2 of pollora/assets (plan: .claude/plans/assets-package.md): the asset classes leave the framework for pollora/assets v0.1.0, which the framework requires. Class names do not change.

Moved to the package (20 files, and the 40 tests that go with them): AssetManager, AssetRegistrationService, AssetRetrievalService, MissingBuilds, AssetContainer, InMemoryAssetRepository, AssetEnqueuer, AssetFile, MissingBuildNotice, the domain models, contracts (ViteManagerInterface, ViteManagerFactory, RuntimeContext, PublicPath…) and exception.

Stays here: AssetServiceProvider, the Laravel implementations of the three contracts (Asset/Infrastructure/Laravel/), ViteManager (Laravel's Vite) and RootAssetManager, the Asset facade, the container contract test and the ViteManager tests.

One change in behaviour kept on purpose: MissingBuildNotice takes its doctor command as an argument (the package says nothing of Artisan); the provider passes php artisan pollora:doctor, so wp-admin shows the same text.

How it was checked

• Full suite sequentially, as CI runs it: 1872 passed (1912 before, minus the 40 moved tests). PHPStan, Pint and Rector clean.
• Rendered HTML of pollora-test compared with the golden captures taken before the extraction, with the package installed from Packagist: identical in build and in hot (theme :5173, plugin :5174 and module :5175 running at once, 12 pages each).
• The package itself: 78 tests, including the architecture test that keeps it free of Laravel, and the same manifests and expectations as the framework's container contract test run against its native resolver.