Add RuboCop cop enforcing Data.define over Struct.new and convert existing usages

[mroderick]

Olle's review on #2855 asked for Data.define instead of Struct.new in ActivityStrip. RuboCop has no built-in cop for this preference, so we should add one and clean up existing usages.

Work

1. Add a custom cop Style/PreferDataDefine (in lib/rubocop/cop/style/prefer_data_define.rb, required from .rubocop.yml). It flags Struct.new sends. No autocorrection: the rewrite isn't safely mechanical (mutability and keyword semantics differ), so each hit gets a human decision.

module RuboCop
  module Cop
    module Style
      class PreferDataDefine < Base
        MSG = 'Prefer `Data.define` over `Struct.new` for immutable value objects.'

        def on_send(node)
          return unless node.receiver&.const_type? && node.receiver.value == :Struct
          return unless node.method?(:new)

          add_offense(node)
        end
      end
    end
  end
end

2. Convert the existing offenses: the 7 Struct.new(:id, :email).new(...) test doubles in spec/helpers/email_header_helper_spec.rb. They are the only usages in the repo, and Data.define(:id, :email).new(1, '...') works with the same positional arguments, so the conversion is mechanical there.

3. Add cop tests (spec/rubocop/... or inline) covering the flag case and a pass case for Data.define.

Notes

• After adding the cop, regenerate .rubocop_todo.yml if needed and confirm CI passes.
• Alternative rejected: a grep-based CI check — less precise and bypasses the existing RuboCop todo workflow.

Context: #2855 (comment)