Ecto-style field schema for collection metadata.

This is the internal representation for:

schema do
  field :title, :string, required: true
  field :date, :date, required: true
  field :draft, :boolean, default: false
  field :tags, {:array, :string}, default: []
end

The public DSL intentionally mirrors Ecto's field/3 shape. Astral uses Ecto schemaless changesets internally to cast external frontmatter, apply field defaults, and validate required fields.