Add Micro::Case#rollback_on_failure alias for #transaction

[serradura]

Summary

• Micro::Case#transaction is shadowed by the attribute reader when a use case declares attribute :transaction (a domain object — e.g. a PaymentTransaction record — that has nothing to do with the database). Inside call!, transaction { ... } then calls the reader and silently drops the block.
• This PR adds alias_method :rollback_on_failure, :transaction as a verbose, unambiguous escape hatch. Same signature, same resolution order (call-site with: > class macro > global default), same transaction(:activerecord) back-compat shim — alias_method makes the equivalence a property of the language, not two implementations to keep in sync.
• The name matches the sibling gem solid-process's rollback_on_failure, so the two APIs read alike.
• Purely additive — #transaction is byte-for-byte unchanged. No new Micro::Case::Check method needed (the alias inherits transaction_owner! / activerecord_loaded! for free).

class RefundPayment < Micro::Case
  attribute :transaction # a domain object, not the helper

  def call!
    rollback_on_failure {
      payment = Payment.create!(transaction: transaction)
      Success(:refunded, result: { payment: payment })
    }
  end
end

Test plan

• New test/micro/case/rollback_on_failure_test.rb covers:
  • alias identity (instance_method(:transaction) == instance_method(:rollback_on_failure))
  • explicit with: / class-macro fallback / global-default fallback resolution
  • rollback_on_failure(:activerecord) { ... } back-compat shim flows through the alias
  • rollback_on_failure(:other) { ... } raises the same ArgumentError
  • real-AR commit-on-success / rollback-on-failure semantics
  • attribute-shadowing regression: a Micro::Case with attribute :transaction calling rollback_on_failure { ... } rolls back correctly; the transaction attribute reader is untouched
• All existing #transaction tests continue to pass unchanged.
• bundle exec rake test (bare bundle): 795 runs / 0 failures.
• bundle exec appraisal rails-8-1 rake test (with ActiveRecord): 1054 runs / 0 failures, both with and without ENABLE_TRANSITIONS=true.

Docs

• CHANGELOG.md — entry under ## [Unreleased] (no version cut in this PR).
• README.md + README.pt-BR.md — lockstep, alias note + example appended to the inline transaction { ... } section. No TOC or Documentation-table changes.

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