DefinedRelationship: BaseRelationship with embedded characteristics, closed at creation

[matt-beanland]

Motivation

AssignedToRelationship conflates a general primitive with the Assigner's domain. We need a clean Provider resource for relationships whose characteristics are set at creation and never changed — once defined, that is it.

Proposed change

Provider.DefinedRelationship

Extends BaseRelationship (source, target, type, timestamps). Adds an embedded Characteristic attribute — a name/value pair embedded directly on the node rather than as separate graph Characteristic nodes:

attribute :characteristics, {:array, EmbeddedCharacteristic}

Actions: create and destroy only. No update, no relate/unrelate. The embedded characteristics are set at creation and closed thereafter — that is the definition.

Contrast with Provider.Relationship which extends BaseRelationship and allows mutable graph-based Characteristic nodes to be added/removed/updated over time.

DefinedRelationship is a general Provider primitive — useful for any relationship whose characteristics are a commitment made at creation time.

Acceptance criteria

• Embedded Characteristic type defined (name + value)
• Provider.DefinedRelationship resource with create/destroy only
• Tests

Follow-on

Assigner refactor (#TBD) — update Assigner to use DefinedRelationship with type: :assigned_to and remove AssignedToRelationship.

[matt-beanland]

The DefinedRelationship has characteristic: {:array, Diffo.Provider.DefinedCharacteristic} which we need to add, an embedded resource which would need to be value: or values, getting us back to Diffo.Type.Value and the dynamic rabbit hole. We can/should do this as we really need to fix Diffo.Type.Dynamic, and we can proceed with Diffo.Type.Value.

[matt-beanland]

We'll actually make DefinedSimpleRelationship, which has a single characteristic with a primitive value. It uses new Diffo.Type.NameValuePrimitive. We also added Diffo.Type.NameValueArrayPrimitive as it felt useful.