Add native ActiveSupport::Notifications hook for transitions

[serradura]

Summary

• New opt-in Micro::Case::Result::Notifications module that publishes one transition.micro_case ActiveSupport::Notifications event per transition, when both transitions and the new config.notifications flag are enabled. Payload exposes :use_case_class, :attributes, :result_type, :result_kind (:success | :failure), :result_data, and :accessible_attributes — plain Ruby values, safe for JSON-encoding subscribers (OTEL, structured logs, StatsD, Rack::MiniProfiler, etc.).
• activesupport stays out of runtime deps. The module probes defined?(::ActiveSupport::Notifications) at require time and redefines publish to a no-op when unavailable, so the transition hot path stays branch-free. Setting notifications = true while AS::Notifications is not loaded raises the new Micro::Case::Error::NotificationsUnavailable (fail-fast at boot rather than silently dropping events). Default is off — existing callers get no behavior change.
• Additive only per the no-breaking-changes policy: Transitions::MapEverything, result.transitions, and Result.new(custom_mapper) are unchanged. CHANGELOG entry under [Unreleased]; both READMEs (EN + pt-BR) gain a top-level Observability section with the Rack::MiniProfiler-step and Rails.logger subscriber recipes, plus the synchronous-subscriber cost warning.

Test plan

• bundle exec rake test — 811 runs, 0 failures (default bundle, AS::Notifications absent → unavailable-path tests run, happy-path tests skip)
• BUNDLE_GEMFILE=gemfiles/rails_8_1.gemfile bundle exec rake test — 1058 runs, 0 failures (AS::Notifications loaded → full happy-path tests run, unavailable-path tests skip)
• ENABLE_TRANSITIONS=false bundle exec rake test — 0 failures (transitions off → no events fire even with config flag on)
• ENABLE_TRANSITIONS=false BUNDLE_GEMFILE=gemfiles/rails_8_1.gemfile bundle exec rake test — 0 failures
• Full bundle exec rake matrix across the Ruby × Rails × ENABLE_TRANSITIONS axes in CI

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