Metadata and construction rules for one signature field.

Prefer the compact signature DSL for ordinary fields. Structured signatures accept each field as an atom, a "name: type" string, {name, options}, or a map. A field map accepts atom or string versions of these keys:

• name (required) and type (an atom or string, default :string)
• kind (:input or :output, normally supplied by the signature)
• desc and prefix
• default, optional, constraints, and language
• metadata for additional JSON-friendly field metadata

Nested types use the string grammar, for example "array[string]" or "array[object]"; tuple forms such as {:array, :string} are not accepted. Constraints are maps such as %{enum: ["red", "blue"]}, %{minimum: 0}, or %{maximum: 1}, keyed by their JSON Schema names; min and max raise. default fills an absent value. optional: true permits omission and represents it as nil.

iex> field = Imp.Signature.Field.new(%{
...>   name: :tags,
...>   type: "array[string]",
...>   optional: true
...> }, :output)
iex> {field.name, field.type, field.metadata.optional}
{:tags, "array[string]", true}

Field declarations describe both sides of the program contract, but input value type mismatches are warning-only at call time for DSPy compatibility. Required input presence and all parsed outputs are enforced.