feat: add type-level ownership checks to the layer validator #28

Description

Some ownership rules can only be checked by looking at types, not at pubspecs or import directives. The ffca-layer-auditor agent covers them with an LLM today.

The validator stays a dependency-free script in this plugin: moving it into Very Good CLI was decided against. package:analyzer is therefore not available, since it would need dart pub get in scripts/ before the hook could run. These checks have to work from source text, the way the import checks from #25 do: read class … implements … declarations and repository method signatures, and resolve type names through the file's imports. A rule that cannot be checked reliably that way stays with the auditor.

Requirements

• Error: a class in {f}_data implements a repository interface from another feature's _domain. The dependency on that domain is allowed, but the data layer implements only its own domain's interfaces.
• Warning: a repository interface in {f}_domain takes or returns a model from another feature's _domain. This is what the Summary pattern in domain.md avoids. Sometimes legitimate, hence a warning (feat: add severity levels and suppressions to the layer validator #21).
• Error: {f}_domain uses dart:ui, Flutter types, or backend types (Firebase, HTTP clients) in its public API. The domain layer is pure Dart (domain.md).
• Each check works from source text with no new dependencies. A rule that would need full type resolution to avoid false positives stays with the auditor, and the issue says which.
• Fixtures and tests for each rule.
• The ffca-layer-auditor agent relies on these results instead of checking the same rules itself.
• All CI/CD checks are passing.
• There is no drop in the test coverage percentage.

Additional Context

Uses the severity levels from #21 and the directive scanner from #31.