Separate publisher process

[gsmetal]

Problem

Concurrent transactions produce artery messages with sequential IDs, but after_commit callbacks fire in unpredictable order. This causes incorrect _previous_index values — consumers see a broken chain and drop valid messages. The previous fix (#82) used FOR UPDATE locks on artery_model_infos to serialize publishing, which worked correctly but caused high lock contention under high concurrency.

Solution

Replace inline publishing with an outbox pattern:

• Messages are inserted into artery_messages directly in model callbacks (after_create/after_update/after_destroy), inside the model transaction. No locks needed.
• A separate Publisher process (bundle exec artery-publisher) polls artery_messages and publishes to NATS in strict id order, guaranteeing a correct _previous_index chain.
• Publisher tracks progress via last_published_id on artery_model_infos, protected by FOR UPDATE (only between publisher instances, not on the hot path).
• Each model gets its own thread in a concurrent-ruby thread pool.

Configuration

Two publishing modes controlled by inline_publish (default: true):

• Inline — publishes from after_commit, no extra process needed. Suitable for development.
• Publisher — set config.inline_publish = false and run artery-publisher. Recommended for production.