Built-in codec for Date.t().
Encodes Date structs to ISO 8601 strings (e.g. "2023-04-01") and decodes
them back. Handles :json and :binary_string formats (returning a binary string)
and :string format (returning a charlist).
Registration
Registered automatically when the :spectral application starts. To use a
different codec for Date.t(), set it in config :spectra, :codecs;
user-configured codecs take precedence over the built-ins.