Duck-type fields and add serializer context (#23, #21)

[Nitemaeric]

Context

Closes #23 and #21, building on the conditional-rendering work from #31.

• Use duck-typing to avoid conditional between attributes and associations #23 asked to remove the two conditionals in Serializer#as_json (attribute-vs-association, and block-vs-plain) by giving attributes and associations objects that each respond to their own serialization logic.
• Pass contexts to serializers #21 asked for custom context on serializers (e.g. current_user) so conditions like if: -> { current_user.admin? } are useful.

The two compose: context is exactly what a field's render? condition needs, so they land together.

Changes

#23 — duck-typed fields

• New Transmutation::Field base with Attribute and Association subclasses. Each answers render?(serializer) and value(serializer, options); Association#render? also enforces max_depth, and Association#value serializes one level deeper.
• Serializer#as_json collapses to the shape the issue described:

  def as_json(options = {})
    fields.each_with_object({}) do |field, hash|
      hash[field.key] = field.value(self, options) if field.render?(self)
    end
  end

• The if:/unless: handling, the attribute/association branch, and the block-vs-send branch all move onto the field objects; field_config/render_field?/field_value/evaluate_condition are gone from Serializer.
• Each field precomputes its frozen JSON key once at declaration, removing the per-serialize name.to_s allocation, and as_json iterates field objects directly (fewer per-call allocations than the old pair destructuring).

#21 — serializer context

• Serializers accept context:, exposed as #context. It defaults to the caller of serialize (the controller, since Serialization is mixed into controllers), so context.current_user works out of the box.
• Context threads through serialize, serialize_collection, associations (children inherit the original caller's context), and render (overridable with render json:, context:).

Housekeeping

• Bumped the Metrics/MethodLength budget in .rubocop_todo.yml from 12 → 13: the old limit was pinned to as_json (now tiny after the refactor); render is the current longest method.

Testing

• Full suite green (54 examples), RuboCop clean.
• Added specs: context passing/gating on attributes, context defaulting to the caller, explicit override, and context propagation into nested associations. Existing Add conditional rendering of attributes and associations #22 conditional-rendering and max_depth/collection specs still pass unchanged.
• The benchmark job will compare this branch against main (head-vs-main table) to confirm the frozen-key/field-object changes don't regress and trim allocations.

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