BaseFeature: typed Ash resource fragment for features

[matt-beanland]

Motivation

Diffo.Provider.Feature is a generic Ash Resource whose characteristics are stored via the generic Characteristic resource (using Ash.Type.Dynamic). Once #128 (BaseCharacteristic) lands, Feature is the natural next step: features become typed resources with real Ash attributes and properly typed characteristic relationships.

Proposed change

Introduce Diffo.Provider.BaseFeature as an Ash Resource Fragment. Feature type modules become Ash Resources that use BaseFeature. BaseFeature itself uses BaseCharacteristic for its characteristics, so feature characteristics inherit the same typing benefits.

DSL impact

feature :name, MyFeatureResource do ... end references a BaseFeature-derived resource. The inline characteristic declarations inside the feature block map to BaseCharacteristic-derived resources on that feature type. The verifier (VerifyFeatures) is updated to check BaseFeature extension.

Instance behaviour simplification

With features as proper resources, set_features_argument and the feature-related wiring in Extension.Feature / TransformBehaviour reduce to standard manage_relationship. The feature/1 and feature_characteristic/2 helper functions generated by TransformBehaviour may simplify or be removed.

Neo4j model

Each BaseFeature-derived resource becomes a distinct Neo4j node label, with its characteristics as typed nodes extending BaseCharacteristic.

Acceptance criteria

• Diffo.Provider.BaseFeature fragment defined, using BaseCharacteristic
• Existing feature type modules migrated to extend BaseFeature
• VerifyFeatures updated to check BaseFeature extension
• Extension.Feature wiring removed; replaced by standard relationship management
• TransformBehaviour / ActionCreate / ActionUpdate further simplified
• Tests updated; existing feature integration tests pass
• DSL cheat sheet and livebook updated

Dependencies

• Requires BaseCharacteristic: typed Ash resource fragment for characteristics #128 (BaseCharacteristic) to be merged first.

[matt-beanland]

Implementation plan

Context

BaseCharacteristic (#128) is done and merged. Feature is currently a generic resource with dynamic characteristics (via Ash.Type.Dynamic). BaseFeature makes features typed resources on the same pattern.

1. Diffo.Provider.BaseFeature fragment

Mirrors BaseCharacteristic in structure: an Ash Resource Fragment that consumer modules extend. Includes:

• Standard id, name, created_at, updated_at attributes
• belongs_to :instance relationship
• Neo4j relate/guard wiring (:HAS incoming from :Instance)
• A marker extension (Diffo.Provider.Feature.Extension) for verifier detection — same pattern as Characteristic.Extension

2. Existing feature type modules

The test support DeploymentClass (and any app-level feature modules) migrate from use Ash.TypedStruct to use Ash.Resource, fragments: [BaseFeature].

3. VerifyFeatures updated

Add a feature_type extends BaseFeature? check alongside the existing characteristic checks — same pattern as the BaseCharacteristic check added in #137.

4. Extension.Feature wiring simplified

set_features_argument and create_features_from_declarations currently create generic Feature records. With BaseFeature, typed features are created via create_typed_features in build_after (mirrors create_typed for characteristics). The dynamic fallback path (Provider.create_feature) is retained for generic Feature resources.

5. TransformBehaviour / build_after

Add Extension.Feature.create_typed_features(result, features()) step alongside the existing create_typed for characteristics.

6. Tests

Existing feature integration tests should pass unchanged. New verifier test for non-BaseFeature type. Transformer test for typed vs dynamic feature detection.

7. DSL cheat sheet and livebook

Document feature :name, MyFeatureModule do ... end form alongside the existing inline form.

Sequencing note

This is a larger refactor than BaseCharacteristic was. Suggest tackling in phases:

1. Define BaseFeature fragment + Feature.Extension marker + verifier check (compile-time safety, no behaviour change)
2. Migrate create_features_from_declarations to typed path
3. Simplify TransformBehaviour once the typed path is stable