Generator: add a Model.schema shorthand for Model.$ack.modelSchema #173

Problem

Models generated by @AckInfer() expose their model-valued schema as Model.$ack.modelSchema. When you pass a model schema to another API, or nest it in a list, you write this a lot, and it reads awkwardly. From a downstream app:

parameters: TaskScreen.$ack.modelSchema,
schema: Ack.list(Task.$ack.modelSchema),
parameters: CreateTaskInput.$ack.modelSchema,

Class-first models already have a short form: AccountSchema.schema is the typed model codec.

Proposal

Generate a static schema on every @AckInfer() model (object, value, union base, and union branch), so the code above becomes:

parameters: TaskScreen.schema,
schema: Ack.list(Task.schema),
parameters: CreateTaskInput.schema,

It would be emitted as static final schema = $ack.modelSchema;. It has the same type as $ack.modelSchema (for an object model, AckSchema<Map<String, Object?>, Task>), it is created once, and it equals Task.$ack.modelSchema.

Collision rule

In Dart, a class can't declare a static and an instance member with the same name, so the generator skips the shorthand when:

• the model has a field whose Dart name is schema;
• the union's discriminator key is schema, because the base declares it as a getter and every branch inherits it;
• the annotated schema variable is itself named schema, because a static schema would shadow the variable the adapter reads.

In those cases Model.$ack.modelSchema still works. The generator neither fails nor renames the member, so no existing model stops generating.

Compatibility

The change is additive and non-breaking. $ack, $ack.schema, and $ack.modelSchema stay as they are. @AckModel() already has XSchema.schema, and the deprecated @AckType() is frozen, so neither changes. The validated value models that the JSON Schema builder generates already declare User.schema as the source validator, and they're unchanged too.