Shared definitions with AsRef

By default, a struct type referenced from multiple places is inlined into the schema at every reference site. Add the zero-arg AsRef() option to a type’s own registration to render it once as "$ref": "#/$defs/TypeName" wherever another registered schema references it instead:

type Shared struct {
    Name string `json:"name"`
}

type Container struct {
    Primary Shared   `json:"primary"`
    Others  []Shared `json:"others"`
}

var _ = jsonschema.NewJSONSchemaMethod(Shared.Schema, jsonschema.AsRef())
var _ = jsonschema.NewJSONSchemaMethod(Container.Schema)

Container’s generated schema gets a $defs object with one Shared entry, and both primary and others.items reference it via $ref instead of repeating its properties.

Nullable references

An AsRef() struct can be the value inside Nullable[T]:

type NullableConfig struct {
    Shared jsonschema.Nullable[Shared] `json:"shared"`
}

var _ = jsonschema.NewJSONSchemaMethod(Shared.Schema, jsonschema.AsRef())
var _ = jsonschema.NewJSONSchemaMethod(NullableConfig.Schema)

The property remains required and accepts either the shared definition or JSON null. Its generated shape is equivalent to:

{
  "anyOf": [
    { "$ref": "#/$defs/Shared" },
    { "type": "null" }
  ]
}

The generator keeps the reachable Shared definition in the containing schema’s $defs; nullable wrapping does not inline or drop it.

Notes:

• AsRef() only changes how Shared is rendered at other types’ reference sites; Shared’s own top-level schema file is unaffected.
• $defs are assembled per generated JSON file, keyed by the type’s bare name. Two distinct AsRef()-registered types reachable in one generation run that share a bare name fail generation with a collision error.
• Recursive or self-referencing AsRef() types are rejected, the same as any other circular reference.

See examples/ref_types for the complete package, generated output, and validation tests.