This PR adds a comprehensive CRE rule for detecting high-severity failures in NATS clusters. The rule is based on the investigation and reproduction of three real-world issues affecting data availability and HA guarantees

Risk & Impact

These are high-severity failure modes that can:

• Cause data unavailability (NATS#6890, NATS#6921)
• Result in silent data loss across nodes (NATS#6929)

Detection of these errors is not possible via standard health checks alone, making this CRE crucial for production observability.

Test Environment

Reproducible test setup (Maintainers invited) : NATS REPRO
Live CRE link: CRE Playground Link

REPRODUCED SCENARIOS:

1. Stream leader failure after node restart due to missing monitor goroutine

• Reproduced via custom Go server cluster with JetStream enabled and stream auto-creation
• Detected via health.log matching:
  "JetStream stream 'app > some-stream' is not current: monitor goroutine not running"

2. Consumer deadlock when using DeliverPolicy=LastPerSubject, AckPolicy=Explicit, and MaxAckPending=1

• Reproduced using nats: latest with a Go pull-based consumer
• Detected via fetch failure logs:
  "Fetch error: context deadline exceeded"

3. Object Store replication drift with no visible errors, but silently inconsistent data

• Verified using nats stream-check --unsynced and raftz/jsz output
• Detected via stream-check logs matching:
  "OBJ_.+│.*UNSYNCED.*"

DEMO

https://asciinema.org/a/gRDKu5PNdw1kdOFXRrhvvS4CN

https://asciinema.org/a/5fVpviwvENVCTvq1vRkUO0zqT

closes #77
/claim #77