feat: check that feature packages belong to their feature #24

Description

Today the validator only checks that a package's name matches its feature folder. The reference implies stronger rules about how a feature's packages fit together, and all of them can be checked from the pubspec graph the validator already builds, with no new dependencies.

Requirements

• Error: {f}_data (and {f}_data_{backend}) must depend on {f}_domain. The data layer exists to implement its own domain's interfaces (Dependency rules in project_structure.md).
• Error: a features/{f}/ folder with a _data package but no _domain package matches no feature archetype (overview.md, headless and presentation-only features).
• Warning: {f}_presentation does not depend on {f}_domain even though {f}_domain exists. A presentation-only feature has no domain, so this only applies when one exists. Depends on feat: add severity levels and suppressions to the layer validator #21; until then, leave this check out. Moved to feat: add severity levels and suppressions to the layer validator #21: implemented in feat(validator): add severity levels and suppressions #30 as presentation-missing-own-domain.
• Error: {f}_domain declares the Flutter SDK (flutter: sdk: flutter). The domain layer is pure Dart (domain.md).
• Each rule has a fixture in scripts/test/fixtures/invalid_workspace/ and a test case, and valid_workspace still passes.
• The Hooks section of README.md lists the new checks.
• All CI/CD checks are passing.
• There is no drop in the test coverage percentage.

Additional Context

Some of these checks are already done by the ffca-layer-auditor agent with an LLM. Moving them into the validator makes them deterministic and lets the hook catch them as soon as a pubspec is edited.