Shared definitions with Ref

By default, a struct type referenced from multiple places is inlined into the schema at every reference site. Add the zero-arg .Ref() chain method 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 _ = polytype.Declare(Shared.Schema).Ref()
var _ = polytype.Declare(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

A .Ref()-registered struct can be the value inside Nullable[T]:

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

var _ = polytype.Declare(Shared.Schema).Ref()
var _ = polytype.Declare(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:

• .Ref() 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 .Ref()-registered types reachable in one generation run that share a bare name fail generation with a collision error.
• Recursive or self-referencing .Ref() types are rejected, the same as any other circular reference.

Migration: NewJSONSchemaMethod(Shared.Schema, AsRef()) is now Declare(Shared.Schema).Ref(). The legacy AsRef() option on NewJSONSchemaMethod/NewJSONSchemaFunc remains supported and source-compatible; it carries a Deprecated: godoc comment naming its fluent equivalent.

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