Add real DM8 failover regression procedure

[skhe]

Summary

• Add a two-phase real DM8 failover check for service-name reconnection and exact DECIMAL(30,8) values before and after promotion.
• Record the isolated Orb primary/standby experiment, correct the earlier inconclusive container-stop interpretation, and document the scope of the verified connection behavior.

Verification

• Local ARM macOS Python 3.10 against an isolated two-node DM8 watcher group: prepare passed; after stopping the primary dmserver and dmwatcher, verify reached the promoted primary and passed both exact decimal checks.
• A first takeover and restart/rejoin were also observed directly: the former primary rejoined as STANDBY, OPEN.
• Python 3.9 syntax compilation and git diff --check passed.

The real failover check remains operator-triggered because GitHub-hosted runners cannot reach the local Orb cluster.