BaseCharacteristic: typed Ash resource fragment for characteristics

[matt-beanland]

Motivation

The provider DSL declaration `characteristic :foo, SomeValueModule` currently assumes `SomeValueModule` is a plain struct used only to seed a `Diffo.Type.Value` dynamic stored on the generic `Diffo.Provider.Characteristic` resource. This means:

• Characteristic values are opaque to Ash — no native filtering, sorting, or expression calculations on typed fields.
• `TransformBehaviour` / `Extension.Characteristic` exist largely to work around the dynamic, creating and relating characteristics via custom wiring rather than standard Ash relationship management.
• The assigner has to spelunk through `Diffo.Type.Value` internals to update individual fields, and the `parent()` / embedded resource approach to liveness doesn't work.

Proposed change

Introduce `Diffo.Provider.BaseCharacteristic` as an Ash Resource Fragment (parallel to `BaseInstance` / `BaseParty` / `BasePlace`). Characteristic type modules become Ash Resources that `use` `BaseCharacteristic`, gaining real typed Ash attributes and standard Ash relationship management.

What stays

`Diffo.Provider.Characteristic` (the existing generic concrete resource, using `Diffo.Type.Value`) is retained as-is. It may itself extend `BaseCharacteristic`, making it one valid option alongside typed resources. The DSL just won't force this approach — it accepts any `BaseCharacteristic`-derived resource, so the caller chooses typed, union, or dynamic depending on what the characteristic shape demands.

DSL impact

`characteristic :foo, SomeModule` continues to work syntactically. `SomeModule` must be a `BaseCharacteristic`-derived resource; `Diffo.Provider.Characteristic` remains available as the dynamic option. `VerifyCharacteristics` is updated to check `BaseCharacteristic` extension.

Instance behaviour simplification

With characteristics as proper resources, creating an instance's characteristics becomes standard `manage_relationship` on the Instance's create action. The custom `set_characteristics_argument` / `create_characteristics_from_declarations` wiring in `Extension.Characteristic` can be removed or simplified. `TransformBehaviour` and `ActionCreate` simplify accordingly.

Neo4j model

Each `BaseCharacteristic`-derived resource becomes a distinct Neo4j node label. `Diffo.Provider.Characteristic` continues to produce `:Characteristic` nodes for the dynamic case.

Acceptance criteria

• `Diffo.Provider.BaseCharacteristic` fragment defined
• `Diffo.Provider.Characteristic` extended from `BaseCharacteristic` (or compatible with it)
• Typed characteristic modules (e.g. `ShelfValue`) migrated to extend `BaseCharacteristic`
• `VerifyCharacteristics` updated to check `BaseCharacteristic` extension
• `Extension.Characteristic` wiring simplified; replaced by standard relationship management where applicable
• `TransformBehaviour` / `ActionCreate` / `ActionUpdate` simplified
• Tests updated; existing characteristic integration tests pass
• DSL cheat sheet and livebook updated

Sequencing

1. This issue (BaseCharacteristic: typed Ash resource fragment for characteristics #128) — foundational
2. Assigner refactor (refactor assigner #120) — `AssignableValue` becomes a `BaseCharacteristic`-derived resource; fields, aggregates, and calculations live directly on it, removing all `Ash.Type.Dynamic` / `parent()` workarounds
3. BaseFeature (BaseFeature: typed Ash resource fragment for features #129) — depends on this issue

[matt-beanland]

What was built:

Diffo.Provider.BaseCharacteristic — Spark.Dsl.Fragment giving typed characteristics a real Ash resource with direct attributes instead of Diffo.Type.Value storage
Diffo.Provider.Calculations.CharacteristicValue — discovers .Value TypedStruct by convention, copies matching fields from the record, returns it as the :value calculation result
Diffo.Type.CharacteristicValue — NewType subtype_of: Ash.Type.Struct, the declared return type for :value calculations on typed characteristics
Three test typed characteristics: Diffo.Test.Characteristic.Card, Shelf, DeploymentClass — each with a .Value TypedStruct using AshJason.TypedStruct for ordered JSON encoding
JSON shape: {"name": "card", "value": {"family": "ARAM", "model": "FANT-B", "technology": "adsl2Plus"}} — name + value, value fields ordered by declaration

What was deliberately deferred:

CharacteristicValue as a full NewType subtype_of: Diffo.Type.Value union — needed before TMF API POST/PUT so characteristics can be accepted as {name, value} and validated by Ash
ApplyCharacteristicValue change module for create actions
Issue #129 (BaseFeature), Issue #120 (Assigner → BaseCharacteristic)

What stayed unchanged:

Diffo.Provider.Characteristic (dynamic) — still handles primitives, arrays, and dynamic values; assigner uses it and passes

[matt-beanland]

updated docs and guidance:

usage-rules.md — characteristics do section now describes BaseCharacteristic-derived resources with a full SpeedCharacteristic + SpeedCharacteristic.Value example
AGENTS.md — added base_characteristic.ex and calculations/characteristic_value.ex to the components listing; updated the DSL example module names; added a common-mistake bullet about using a plain TypedStruct as a characteristic target
documentation/how_to/use_diffo_provider_extension.livemd — ClusterValue/GPUValue TypedStructs replaced with ClusterCharacteristic/GpuCharacteristic BaseCharacteristic resources + .Value structs; update :define removed from Cluster (no dynamic characteristics); GPU's update :define now only handles the dynamic :cores AssignableValue; typed :gpu characteristic is updated directly via Compute.update_gpu_characteristic!; domain adds ClusterCharacteristic and GpuCharacteristic resources with their update functions