Closes #2.

Adds Micro::Case::ActiveJob — a built-in, opt-in async runner. Purely additive; the public API of u-case stays frozen and the gemspec is unchanged (activejob remains an optional, host-provided dependency).

Summary

• MyCase.async.call(input) / MyCase.later.call(input) — enqueue any Micro::Case subclass via ActiveJob. Per-use-case job subclass is built lazily and named <UseCase>::Job so it serializes across enqueue/dequeue.
• active_job do … end class macro — eager DSL evaluated at class-body load time. Narrow surface: key, retry_on, discard_on, after_discard, default_options, around_perform, after_transaction_commit. Unknown methods raise NoMethodError with the valid-method list for typo protection.
• Micro::Case::ActiveJob::Registry — thread-safe Mutex-backed mapping of stable string keys to use-case classes. Auto-registers klass.name => klass on first call so day-one adopters don't need a DSL declaration. Explicit key '…' makes renames safe: the payload carries the key, not the constant name.
• Micro::Case::ActiveJob.batch(pairs) — uses ActiveJob.perform_all_later on Rails 7.1+, falls back to per-pair enqueue on older Rails.
• Micro::Case::ActiveJob::Error (raised on the worker when raise_on_failure: true and the result is a failure; exposes .result) and Micro::Case::ActiveJob::UnknownKey (raised when a payload's key doesn't resolve).
• Micro::Case.config.strict_registry (default false) — disables the safe_constantize fallback for registry misses.

ActiveJob features exposed declaratively: retry_on / discard_on (Rails 5.1+), after_discard (Rails 7.1+; logs once and no-ops on older Rails), enqueue_after_transaction_commit via after_transaction_commit :always|:never|:default (Rails 7.2+; logs + no-ops on older Rails), around_perform, default_options. GlobalID-coded inputs work transparently — the runner does not pre-coerce input on the worker side.

All 10 new internal argument validations route through Micro::Case::Check::Enabled / Disabled (matching the convention introduced in 5.4.0), so they participate in config.disable_runtime_checks = true.

Open-question resolutions

• Bulk enqueue lives on Micro::Case::ActiveJob.batch (not Micro::Case.async_batch) — keeps Micro::Case's class-method namespace tight.
• Inheritance: subclasses inherit job subclass reference, default_options, and registered key. A subclass that redeclares active_job do … end must use a fresh key — enforced by the registry's duplicate-key check.
• Auto-registration on first async call: enabled.
• Per-enqueue job_options: wins over DSL default_options (matches Rails' set overriding queue_as).
• strict_registry default: false (friendlier for early adoption).
• DSL evaluator named Micro::Case::ActiveJob::DSL.
• Implementation lives in a single file (lib/micro/case/active_job.rb).

Test plan

• bundle exec rake test — 832 runs, 8246 assertions, 0 failures.
• bundle exec appraisal rails-8-1 rake test — 1079 runs, 0 failures.
• ENABLE_TRANSITIONS=false bundle exec appraisal rails-8-1 rake test — 745 runs, 0 failures.
• Verify the gem still loads without activejob (core lib/micro/case.rb does not require any active_job code; check.rb only adds methods, no AJ constant references).
• 37 new tests cover: basic enqueue, job_options threading, raise_on_failure, rename-via-registry, auto-registration, duplicate/unknown keys, DSL methods (retry_on, default_options, around_perform, typo protection), inheritance, batch, to_proc composition, strict mode, and the check toggle for each new method.
• Full matrix run (bundle exec rake matrix) against the CI grid — recommended before merge.

Out of scope (not bumped here)

• Version bump. Per CLAUDE.md, this would be a minor bump (purely additive, no dependency-floor change), but the bump is intentionally not done in this PR — release tagging is a human step. The CHANGELOG entry sits under [Unreleased] until then.
• README Documentation / Compatibility table updates — those bump only on release.

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